Epoxy over PVA

[dkuzminov]

I'm gluing planks using a PVA-like glue (the manufactorer says it is "advanced proprietary polymer"). This is an easy solution for fixing planks to the bulkhead without nails. However I'm not satisfied with the stiffness of planks, so I plan to streighten the hull with epoxy. The question I have is how does epoxy adhere to PVA seams. What are the caveats, should I clean the planks from PVA where possible, or that is not needed?

[vaddoc]

How  and where are you planning to apply the epoxy?

If you plan to fillet the joint of bulhead and plank from inside of the hull, best to thicken the epoxy with talk powder (or anything else, i ve used ground coffee in the past) to consistency of peannut butter and apply. It should work fine. Epoxy is a bit messy though. Uncured epoxy from hands cleans with vinegar but best to wear gloves

Edited April 24, 2020 by vaddoc

[dkuzminov]

12 minutes ago, vaddoc said:

How  and where are you planning to apply the epoxy?

If you plan to fillet the joint of bulhead and plank from inside of the hull, best to thicken the epoxy with talk powder (or anything else, i ve used ground coffee in the past) to consistency of peannut butter and apply. It should work fine. Epoxy is a bit messy though. Uncured epoxy from hands cleans with vinegar but best to wear gloves

I'm applying epoxy to the inner surface of planks fastening them into a robust shell. Planks would bend while sanding otherwise.

What does thickening give in this case to compare with raw epoxy?

The advice about ground coffee is very surprising. Is there any benefit of ground coffee to compare with sawdust?

[Moxis]

I have used milled fiberglass mixed with epoxy to a gel like stuff and applied it inside the hull. When cured it strengthens the hull considerably and makes it watertight.

 

https://www.amazon.com/Great-Planes-Milled-Fiberglass-Ounces/dp/B001BHEGRO

[wefalck]

'Micro-balloons' (https://en.wikipedia.org/wiki/Glass_microsphere) are the standard filling material for epoxy and other resins.

 

If the model is going to be a stationary one, not exposed to water, the stiffening effect could be probably achieved without having to mess around with epoxy by just liberally painting on PVA glue.

 

However, the more common procedure would be to use filler blocks, normally inserted between the bulkheads before the planking is applied. If you use styrofoam or other hard foam, this could be done easily retrospectively, again using PVA rather than some messy two-component resin (which would not be compatible with styrofoam anyway).

[Jaager]

You have already planked a POB hull?

The moulds are few in number and widely spaced?

This is a two layer planked hull and you are shaping the first layer?  The planking wood is thin and flexes under force?

 

As Wefalck writes,  the usual practice is to use filling material between the moulds, shaped to hull lines before any planking is applied.  Done correctly, only one layer of planking is then required.

It has been my impression that on a two layer hull, the first layer is relatively stiff and the display layer id often a thin decorative species veneer.  (To my eye, often a species with out of scale grain and open pores, but never mind that.)  Did you plank the first layer using the veneer?

 

Now, to your present situation:  epoxy sounds like a messy and much less than ideal solution.  I would obtain a supply of wood veneer that is thin enough to bend inside the 90 degree arc amidship. 

Use a sharp knife to cut popsicle stick wide or narrower strips and PVA glue a layer or two inside in a vertical orientation.  No compatibility problems.

[dkuzminov]

17 minutes ago, Jaager said:

You have already planked a POB hull?

The moulds are few in number and widely spaced?

This is a two layer planked hull and you are shaping the first layer?  The planking wood is thin and flexes under force?

 

As Wefalck writes,  the usual practice is to use filling material between the moulds, shaped to hull lines before any planking is applied.  Done correctly, only one layer of planking is then required.

It has been my impression that on a two layer hull, the first layer is relatively stiff and the display layer id often a thin decorative species veneer.  (To my eye, often a species with out of scale grain and open pores, but never mind that.)  Did you plank the first layer using the veneer?

 

Now, to your present situation:  epoxy sounds like a messy and much less than ideal solution.  I would obtain a supply of wood veneer that is thin enough to bend inside the 90 degree arc amidship. 

Use a sharp knife to cut popsicle stick wide or narrower strips and PVA glue a layer or two inside in a vertical orientation.  No compatibility problems.

So far I've done 5 rows of planks and plan to straighten them incrementally because it would be difficult to access the inner surface later.

This is a single-layer model with 2mm-thick sapelli planks and 8cm-wide moulds. Planks flex under force if not streighten with epoxy, but are much stiffer if a thin layer of epoxy is applied.

There is one more reason why I've started using epoxy to streighten the hull. The tips of planks are fixed on a 6mm plywood, thus each tip covers just a 3mm of plywood. That is enough to fix the plank, but if there is a little torsion, PVA is not enough to fix the tip perfectly. Adding epoxy to the corner solves the issue.

[wefalck]

Sounds a bit strange. Perhaps you want to post a couple of pictures ?

[vaddoc]

😉 ground coffee is essentially wood dust! 

Epoxy is strong but brittle. For used as glue when gap filling is needed, for filleting, for laminating, it needs to be thickened. It increases the strength, reduces brittleness and makes it much less runny. If sanding is needed, it makes sense to use a filler that sands easily. I ve worked a lot with epoxy and I like talk powder.

I think that the way your idea could work would be to mix some epoxy with talk powder, wood dust or whatever similar and actually glue thin strips of wood on the inside of your planking. This would resemble a "cold mould" construction and should be pretty solid. It will be lots of messy work though, maybe best to go for filler pieces or more bulkheads

[dkuzminov]

10 hours ago, wefalck said:

Sounds a bit strange. Perhaps you want to post a couple of pictures ?

Ok, here are the pictures.

The first picture shows the tips of the planks glued with PVA to the bulkhead. The thickness of the plywood is 6mm. This seam is almost invisible.

The next picture shows a bad seam where the planks bend significantly. You may see that two planks (the middle row) have torsion, and don't form the surface, partially because of the elasticity of PVA.

That is the reason why I'm streightening the inner surface with epoxy (you may see epoxy glittering from the inner surface of planking):

[wefalck]

Looking at your bulkheads, it appears that the real problems is, that they have not been faired. I can see the discolouring from the laser-cutting across their whole width. Of course, the planks will make kinks and buckle then, when bent across the sharp edge of the bulkhead.

[dkuzminov]

2 minutes ago, wefalck said:

Looking at your bulkheads, it appears that the real problems is, that they have not been faired. I can see the discolouring from the laser-cutting across their whole width. Of course, the planks will make kinks and buckle then, when bent across the sharp edge of the bulkhead.

No, that is not the case, I've shaped the bulkheads pretty well. The one that has this discolouring is the central bulkhead where the planks are perpendicular.

[wefalck]

Well, if the planks are too thin, this effect can occur as well, when they are bent across quickly changing radii. The best solution would be to fill the spaces between the bulkheads and then fair again the whole thing.