Jump to content
Drazen

Cracking of wood due to strong humidity deviations – need help

Recommended Posts

2 hours ago, paulsutcliffe said:

my cheerful hull below after a year in the garage the wood is holly and was dry and seasoned but has obviously swollen after fitting and painting with watered down acrylic 

 

  Ah nuts, that is really disappointing to see the damage after all of your efforts to produce a nice hull. I have no doubt that you can sort it when the time comes but still....

Share this post


Link to post
Share on other sites
3 hours ago, paulsutcliffe said:

Expansion can then become just as big a problem though as the shrinkage, my cheerful hull below after a year in the garage the wood is holly and was dry and seasoned but has obviously swollen after fitting and painting with watered down acrylic

You betcha! I avoid water-based acrylics as much as possible for exactly that reason, even though I seal my wood with white shellac before painting. Sometimes I cheat and thin them with alcohol, if that works on a particular brand. Airbrushing with alcohol solvent allows the solvent to evaporate quickly. Trying to airbrush paint thinned with water results in a wet mess every time. Oil-based, solvent-thinned "modeling" paints are getting harder to find, although the fairly new acetone-based Truecolor acrylics, claiming to be equivalent to the old Floquil, are getting great reviews. Artists' oils (Grumbacher, Windsor and Newton) are fine if you don't mind conditioning your own with acetone, turpentine, Japan drier, Penetrol, and linseed oil, raw or "boiled," provided the pigment is ground finely enough for modeling scale purposes.

 

If it's any consolation, Paul, the seams on your model look quite like the seams on many newly built carvel planked boats I've seen. In fact, in full-size construction, it is expected that the swelling of the planks after launching will cause the stopping in the seams to bulge outwards exactly how the putty or filler you apparently placed between the seams on your model has done. On full-sized boats, the bulging stopping is sanded fair at the next haul out and paint job looks to be doing. This phenomenon can continue with a new boat for two or three haul out cycles before the planking "settles down," or stabilizes somewhat. Of course, if a boat is on the hard for a long period of time and dries out significantly, the problem will usually arise again upon the following launch.

 

The same problems occur in building models and full-sized wooden boats. Wood is a dynamic medium, always moving, to one degree or another, depending upon temperature and humidity changes. The degree of that movement depends also upon the species of wood and the grain orientation of the wood. In the case of the model in this thread, Lime ("Basswood" in the US) shrinks 6.6% radially and 9.3% tangentially, Pear shrinks 3.9% radially and 11.3% tangentially, and Maple shrinks 4.8% radially and 9.9% tangentially. These percentages are for drying from a live cut. Once fully dried, the "movement" factor due to changes in relative humidity may vary from "live cut" shrinkage, and is referred to as the species' "stability."  Stability will generally be a lower percentage of movement than "drying" from a live cut and, as said, will vary from species to species. The ratio of movement radially and tangentially after initial drying will likely be proportional to the stability of a species.  The Wood Database is an excellent resource. A handy hyperlinked version of their print book can be found at https://www.wood-database.com/basswood/ 

 

It's important to remember that the actual distance of movement multiplies relative to the size of the piece. Thus, a one quarter-inch wide plank might move +/- 4% with wide fluctuations in humidity, which would be nearly negligible over the width of a quarter-inch plank, but if, say, sixteen quarter-inch planks are butted against each other, the total movement of the four inch width of quarter-inch planking will move in total almost an eighth of an inch, which, if it's shrinking, is a lot more than the elasticity of the wood is going to endure without splitting, and if it is expanding, will be enough to induce stresses in the structure to which the planks are attached to cause catastrophic failures. In full-sized vessels, transversely cracked frames are a frequent result of planking expansion, often caused or exacerbated by driving caulking too hard or filling seams with inflexible, hardened stopping or splines of harder wood species. Regardless of species, wooden boats are always moving structures, some more than others for a variety of reasons. Some, the Vikings' longboats as a classic example, are even intentionally engineered to flex in operation. Models are different only in a matter of scale. What doesn't "scale," however, are the laws of hydraulics. The amount of pressure created by the water in the wood doesn't vary according to the size of the wood. The water in a tiny piece of wood expanding isn't going to compress any more than the water in a large piece, even if the amount of expansion is proportional to the size of the piece.

 

Sealing wood by coating or impregnating it with some water-resistant material will slow, but not stop, movement because it retards the absorption and evaporation of moisture in the wood. Much as many have tried, there is nothing found as yet that totally "encapsulates" wood. Indeed, the unintended consequence of limiting wood's ability to "breathe" in the marine environment can often create a more favorable environment for decay fungi. Balsa-cored fiberglass decks were once thought to be an innovative solution to achieving lightweight panel stiffness until it was discovered that "water will always find a way" and moisture entering through poorly sealed fastener holes and such soon caused the balsa cores to decompose to rotten mush. As models won't be living in the water, the problem is less severe. One of the best sealers is also one of the oldest. A coating of white shellac is an excellent sealer for model parts. Coating planks with white shellac ("white" because it's the colorless type versus "orange" shellac, which is unbleached.)  It won't prevent the wood from moving ever, but it will slow down the absorption and evaporation cycles causes by ambient humidity considerably. The fewer movement cycles, the less stress is placed on the structure over time.

 

I'm sure a lot of experienced modelers already know this. Those who have experience in full-size wooden boatbuilding certainly are familiar with these principles. These comments are offered for those who are in their modeling "learning curve" where others of us were decades ago. As for myself, most of what I know about this was learned from experience, A mistake is a lesson that isn't soon forgotten. 

 

 

Share this post


Link to post
Share on other sites
9 hours ago, paulsutcliffe said:

Expansion can then become just as big a problem though as the shrinkage, my cheerful hull below after a year in the garage the wood is holly and was dry and seasoned but has obviously swollen after fitting and painting with watered down acrylic

 

I saw that in I guess your build log for it? I can't look at those pictures, it's like a scene from a 1960s Driver's Ed movie. Not to mention that's a few dollars of holly unless you milled it yourself. In which case the time and machines cost a lot. Either way a seriously bad day.

Share this post


Link to post
Share on other sites

Bob Cleek:

 

Acoating of white shellac is an excellent sealer for model parts.”

 

When would one apply the shellac?  Must be after planking, since we need moisture in the planks to form to the hull ( less so for a deck). But, If after planking, which is after adding blocking between ribs, how does sealer get to the underside of the plank?  The blocking should be coated as well, before planking?

 

Or, is the idea that you seal as much as you can , and continue to hope?

Share this post


Link to post
Share on other sites
3 hours ago, Srodbro said:

When would one apply the shellac?  Must be after planking, since we need moisture in the planks to form to the hull ( less so for a deck). But, If after planking, which is after adding blocking between ribs, how does sealer get to the underside of the plank?  The blocking should be coated as well, before planking?

 

Or, is the idea that you seal as much as you can , and continue to hope?

The latter. You do the best you can sealing, depending on the circumstances, and hope for the best. Usually, an element, such as a deckhouse, can be sealed as it's completed and before installing. Sealing is best done just before painting, or what would be painting if not left natural. Planking, of course, is sealed after the topsides are faired and the job is done. The shellac should be thin so that it soaks into the wood readily and doesn't leave any visible build-up after it's dry.

 

There will always be ambient moisture in the wood. I'm not sure, however, from where the misconception comes that it is necessary to wet wood to bend it, though. It's the heat that's necessary. Moisture, whether it be boiling water or steam, is simply one convenient way to convey heat from its origin to the wood you want to bend. The gondola builders in Venice actually bend their planks over a live fire, taking care not to burn the wood.

 

Some experienced modelers use other types of sealers. Minwax "wipe on" finishes seem popular. I'm not sure how Minwax compares to shellac in terms of a moisture barrier, but it will serve the same sealing effect to one degree or another.  It would provide a soft satin finish, and bring out the wood's color and figuring, better than shellac as it's intended as a visible finish. Shellac is capable of doing the same, but that requires a lot of hand rubbing and repeated applications, as is done with fine classic furniture, and their delicacy makes miniatures poor candidates for a traditional "French polished" finish. I can't comment on the archival properties of Minwax and other modern and often synthetic finishes. Like many materials available today, they simply haven't been around long enough for us to know. Shellac's longevity, on the other hand, is well established by the archaeological record.

1 hour ago, druxey said:

I suspect that humidity control is far more effective than any sealer.

Absolutely! That's the first line of defense. Moisture always seeks an equilibrium. The value of a sealer is that it slows the rate of absorption and evaporation and so "levels out" the cycling between high and low humidity. That retards the amount of movement. The cyclic movement of the parts will weaken the structure over time. The less movement, the longer the structure stays strong and tight.

Edited by Bob Cleek

Share this post


Link to post
Share on other sites

Well, spraying very thinned shellac is possible with my airbrush(es). I can do several thin layers after the first one has been set and soaked into the wood, the next one comes to make the sealing more tight.

What actually  is the problem with airbrushing, is that you have an airflow which must hit the surface in order to leave paint there. The areas, where the spray is not circulating (e.g. corners) will be covered not so well. But, I must try with a stronger amount of medium. I can make a big mess inside my gunports since they are more or less closed on the lover decks.

I will leave the 50% humidity till spring and when it starts to go over the 50% by itself, I will take my gun (airbrush-gun I mean) and do this job.

 

Bob, a very nice explanation., Thank you!

 

Drazen

Share this post


Link to post
Share on other sites
4 hours ago, Drazen said:

Well, spraying very thinned shellac is possible with my airbrush(es). I can do several thin layers after the first one has been set and soaked into the wood, the next one comes to make the sealing more tight.

An airbrush will work, but I use a brush simply because I don't have to worry about masking and overspray or getting into the cracks and corners. I only apply a single coat, as thinned shellac will penetrate bare wood rather well. Adding more shellac on top of what's already soaked into the wood is really only gilding the lily. Thin as the shellac is, or should be, there aren't any problems with brush strokes mucking up the smoothness of the surface. It's barely noticeable when dried... or should be.

Share this post


Link to post
Share on other sites

Bob,

 

I do also apply shellac by brush on the areas which are accessible to the hand-brush.

The whole idea on airbrushing shellac was to protect the inner areas of the ship by airbrushing through gunports. The wood has not been protected from inside, but only from outside (this by hand brushing shellac or Golden GAC100). Still, the ship hull is getting or loosing moisture from inside and this happens for instance through the gunports. I think the most of the harm has been happened by non-protected inside and exchange through gunports.

I cannot reach every corner of the inside of the ship by brush - by airbrush probably better.

 

Please, check the previous photos of the ship in different stages in order to understand what I mean. Here again also the link to my build:

 

Drazen

Edited by Drazen

Share this post


Link to post
Share on other sites

My hms Sirius model is in the same garage as the cheerful Hull and I have had no problems with cracking or shrinking, it has been out there for years now, even through our latest hottest summer for years, however each layer as I have been building from the orlop up has been coated in layers of Tung oil thinned with spirits for the first coat 50/50

I'm not sure if this is what's made the difference to this model or not

Regards

Paul

Share this post


Link to post
Share on other sites
9 hours ago, Drazen said:

The whole idea on airbrushing shellac was to protect the inner areas of the ship by airbrushing through gunports. The wood has not been protected from inside, but only from outside (this by hand brushing shellac or Golden GAC100). Still, the ship hull is getting or loosing moisture from inside and this happens for instance through the gunports. I think the most of the harm has been happened by non-protected inside and exchange through gunports.

I cannot reach every corner of the inside of the ship by brush - by airbrush probably better.

Got it. The airbrush would be a better option. Applying the shellac to areas before they are closed up will prevent that in the future, of course.

Share this post


Link to post
Share on other sites

Let me now explain my strategy how to deal with my problem.

 

1..Get the humidity to approx 50% and keep it there by humidifier in winter and dehumidifier in summer.

 

2. After the wood has been acclimatised (e.g. 2 months), I am going to spray few very thinned layers shellac through the gunports. For this, I will probably close all, other openings to get the area filled well. I want to get shellac also in the corners inside of the hull.

 

3. Than, depending on how much the cracks went back/recovered, I am going to pour epoxy into the gaps where wood cracked.

 

4. Repairing of the decks if needed. I have some vital parts on the decks damaged (frames of the gratings) which are not so easy to be made again. Better would be not to need to repair this. Let us see.

 

What I am still unsure is if it makes sense to spray thinned epoxy instead of shellac inside of the hull. Epoxy would make the whole thing very solid, but I must check if there are good thinner in order to be able to clean the airbrush afterwards. Also, the toxicity of epoxy is an issue. But, for this I have a very good mask which has a very good filter build in.

 

Any comments on this?

 

Drazen

Share this post


Link to post
Share on other sites

Hi Drazen , is there a way you could thin the bulkiness of wood off the hull through ports? Like making your structure more as POB (planks on bulkheads) instead of solid hull you currently have?  My belief is that removing material from the structure, making it weigh less, reducing the structure to only a skeleton of a hull - same as real ship hulls were made - this makes the structure less susceptible  to humidity variations. Imagine a real ship made in the ship yard. They made it in dry air environment and when ready they dropped whole thing in the water. I think main trick is making ships of THIN  wood as real ships were made. Protecting with chemicals may help but .... for this-  there is no proven theory.

Share this post


Link to post
Share on other sites
5 hours ago, Drazen said:

3. Than, depending on how much the cracks went back/recovered, I am going to pour epoxy into the gaps where wood cracked.

 

Filling cracks with anything that is as inflexible as epoxy resin isn't a good idea. If and when the wood swells up again, it will be pressing against that inflexible cured epoxy and cause greater cracking further down the crack. You'd be much better off to replace the cracked piece entirely, if at all possible, with properly dried wood of the same species.

 

5 hours ago, Drazen said:

What I am still unsure is if it makes sense to spray thinned epoxy instead of shellac inside of the hull. Epoxy would make the whole thing very solid, but I must check if there are good thinner in order to be able to clean the airbrush afterwards. Also, the toxicity of epoxy is an issue. But, for this I have a very good mask which has a very good filter build in.

As you seem to realize, atomizing epoxy resin isn't a good idea. Inhaling it isn't at all recommended. (And this advice is coming from a guy that isn't a sissy about such things.) Epoxy isn't going to make things much more solid unless you build up a rather thick coating and standard cured epoxy resin is quite brittle. (WEST Systems epoxy markets a brand of epoxy known as "G-flex" which cures to a somewhat flexible state. It's now being used as an adhesive in full-sized wooden boatbuilding, but I don't think would do you any good in this instance. As a structural material, epoxy has to be used in a matrix of stronger material, such as glass or carbon fiber fabric or matting. Again, I'm afraid a "do over" of the damaged parts is what is required to restore your model in this case.

 

Most epoxies can be thinned with alcohol or acetone, but I shudder to think what gunked up epoxy would do to an airbrush!

Share this post


Link to post
Share on other sites

I agree with Bob.  Never mind that the air brush will be ruined, but your lungs and nervous system will be too.

 

Controlling the humidity is the best strategy, then replacing the cracked planks should 'solve' the problem.

 

BTW, Longridge's HMS Victory develop cracks in its hull planking……..        Duff

Share this post


Link to post
Share on other sites

Agree. luthiers use epoxy resin all the time as their grain-filler, but they spread it squeegee-style with plastic spreaders, I've never heard of anyone trying to spray it.

 

Also Bob is correct that you need to account for the relative flexibility of the glue, epoxy once fully cured is very rigid and replacement of the wood is a better idea. If it's not possible, use yellow glue as it remains flexible to a degree after curing. There are also some (apparently) good newer CA glues that remain flexible after curing, but they don't have much of a track record yet, and that's important in a model intended to last for decades at least.

Share this post


Link to post
Share on other sites

The bottom line to all this discussion is, WOOD WILL MOVE. All you can do is minimize this by humidity control and not making any part too large. As suggested, painful as the thought may be, it is now a matter of re-doing some of the work by replacing the fractured pieces.

Share this post


Link to post
Share on other sites
On 2/10/2019 at 12:41 AM, Y.T. said:

Hi Drazen , is there a way you could thin the bulkiness of wood off the hull through ports? Like making your structure more as POB (planks on bulkheads) instead of solid hull you currently have?  My belief is that removing material from the structure, making it weigh less, reducing the structure to only a skeleton of a hull - same as real ship hulls were made - this makes the structure less susceptible  to humidity variations. Imagine a real ship made in the ship yard. They made it in dry air environment and when ready they dropped whole thing in the water. I think main trick is making ships of THIN  wood as real ships were made. Protecting with chemicals may help but .... for this-  there is no proven theory.

I am afraid this is in that stage no more possible. Please, check my build log. Changing basic structure would mean starting building from the beginning. But, please see my build process and if you have any ideas, maybe you see it from another perspective.

 

The solid hull was meant to get a solid ground base for the planking. This showed to be of great benefit. Still, it brings some drawbacks with it, as you see - the sensitivity to humidity. The idea of solid hull was not mine and not new. Some top builders are using it for their ships all the time. It seems that several "small" differences did accumulated and made the system to "tip over": I definitely have extreme variations in my work room, the hull is relatively large, I did not protect inside of the hull during building the hull, De 7 Provincien has in the middle a large area with gratings (so, there is an "U-shape" which means that the upper part wants to widen when the wood contracts)... Just some aspects.

 

Drazen

Share this post


Link to post
Share on other sites
On 2/10/2019 at 5:24 AM, Bob Cleek said:

Filling cracks with anything that is as inflexible as epoxy resin isn't a good idea. If and when the wood swells up again, it will be pressing against that inflexible cured epoxy and cause greater cracking further down the crack. You'd be much better off to replace the cracked piece entirely, if at all possible, with properly dried wood of the same species.

 

As you seem to realize, atomizing epoxy resin isn't a good idea. Inhaling it isn't at all recommended. (And this advice is coming from a guy that isn't a sissy about such things.) Epoxy isn't going to make things much more solid unless you build up a rather thick coating and standard cured epoxy resin is quite brittle. (WEST Systems epoxy markets a brand of epoxy known as "G-flex" which cures to a somewhat flexible state. It's now being used as an adhesive in full-sized wooden boatbuilding, but I don't think would do you any good in this instance. As a structural material, epoxy has to be used in a matrix of stronger material, such as glass or carbon fiber fabric or matting. Again, I'm afraid a "do over" of the damaged parts is what is required to restore your model in this case.

 

Most epoxies can be thinned with alcohol or acetone, but I shudder to think what gunked up epoxy would do to an airbrush!

When I was a student, a very good friend and me were restoring an old small sailing boat (Finn class). It was wooden and I can remember, we have had a diluting/cleaning medium for epoxy and for cleaning our tools. However, we did not use airbrush, but manual tools to apply epoxy. The whole ship hull was soaked with epoxy to prevent water coming in and to stiffen the structure. When sailing regattas (what we did), it was very important to have a very stiff hull which does not flex. We did not have money to buy a new boat made of new materials, but had to be innovative. 🙂

 

I think, I will protect inside of the hull with shellac than.

 

Drazen

Share this post


Link to post
Share on other sites
On 2/10/2019 at 11:18 PM, druxey said:

The bottom line to all this discussion is, WOOD WILL MOVE. All you can do is minimize this by humidity control and not making any part too large. As suggested, painful as the thought may be, it is now a matter of re-doing some of the work by replacing the fractured pieces.

Yup! The humidity control, i already do.

 

Drazen

Share this post


Link to post
Share on other sites
On 2/10/2019 at 9:50 PM, vossiewulf said:

Agree. luthiers use epoxy resin all the time as their grain-filler, but they spread it squeegee-style with plastic spreaders, I've never heard of anyone trying to spray it.

 

Also Bob is correct that you need to account for the relative flexibility of the glue, epoxy once fully cured is very rigid and replacement of the wood is a better idea. If it's not possible, use yellow glue as it remains flexible to a degree after curing. There are also some (apparently) good newer CA glues that remain flexible after curing, but they don't have much of a track record yet, and that's important in a model intended to last for decades at least.

What is a "yellow glue"? I know the "white glue" should be PVA-glue what carpenter are commonly using.

We have in Europe some sometimes other names and brands.

 

Just a note: Be careful with CA-glue + maple! It will leave a yellow film/shade near the bonds, which may only be removed by leaving the model staying on the sun for several weeks (something like letting plastic models near the window after the decals got a yellow shade). Wit e.g. pear wood, this is not the problem - no yellowing.

 

Drazen

Share this post


Link to post
Share on other sites
1 hour ago, Drazen said:

What is a "yellow glue"?

High strength PVA glues designed for woodworking. Here the most common is Titebond.

 

Are you sure the yellow shade from CA isn't a very thin layer of CA near the joint? I've seen that with several light woods, but only when there was an actual layer of CA on the surface. 

Share this post


Link to post
Share on other sites

OK, I think I know which glue you mean. I am using the D3 glue for professional and industry use, not the one which can be both in common building supplies stores. It is water resistant, but not meant for outside being left in the rain. There are wood glues from D1 as the lowest resistance, up to D4 with complete resistance on water. Since my model is not meant to swim, the D3 is more than enough.

 

Concerning CA: Please, please see the photos. A single water spout has been made here of 8 pieces and glued with CA gel glue from the back side. No glue on the visible side. I guess the fumes did the yellowing effect. CA fumes are strong and can ruin for instance a canopy on the plastic plane model. I have had many of this on gratings and got it away by letting them behind the window glass. To remove the effect here, I will try with an UV-lamp.

 

Drazen

20190213_003232.jpg

20190213_003312.jpg

2014-02-26.JPG

Share this post


Link to post
Share on other sites
3 hours ago, Drazen said:

Concerning CA: Please, please see the photos. A single water spout has been made here of 8 pieces and glued with CA gel glue from the back side. No glue on the visible side. I guess the fumes did the yellowing effect.

Interesting, you're right but I have honestly never seen that, and I've used CA on maple before. I suggest trying a different CA glue if you can and see if it does the same thing before you totally rule it out for future use.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

About us

Modelshipworld - Advancing Ship Modeling through Research

SSL Secured

Your security is important for us so this Website is SSL-Secured

NRG Mailing Address

Nautical Research Guild
237 South Lincoln Street
Westmont IL, 60559-1917

About the NRG

If you enjoy building ship models that are historically accurate as well as beautiful, then The Nautical Research Guild (NRG) is just right for you.

The Guild is a non-profit educational organization whose mission is to provide support to our members in their efforts to raise the quality of their model shipcraft.

The Nautical Research Guild puts on ship modeling seminars, yearly conferences, and juried competitions. We publish books on ship modeling techniques as well as our world-renowned quarterly magazine, The Nautical Research Journal, whose pages are full of articles by master ship modelers who show you how they build those exquisite details on their models, and by maritime historians who show you what details to build.

Our Emblem

Modelshipworld - Advancing Ship Modeling through Research
×
×
  • Create New...