How to strengthen wood?

[bruce d]

I am doing things with Swiss pear that includes some thin sections, typically 2mm x 3mm. The grain is sometimes with me and sometimes not. Not surprisingly, the narrow pieces with cross-grain are fragile.

Any thoughts on strengthening this wood? I am thinking of what flying model builders do when they smear CA across balsa and increase it's strength substantially but that isn't going to work: I need to apply a finish. And no, sadly I can't laminate pieces to create plywood.

Thinking out loud here ... does shellac add any strength to wood? 

I'm stumped.

 

Bruce

[knightyo]

Hi Bruce, I'm in the same boat.  Also using swiss pear, also going to shellac and nervous about the strength of some of my very small yards. 

 

Alan

[Bob Cleek]

As with all structural materials, wood has its structural limitations which vary from species to species. There is very little that can be done to treat wood in order to enable it to exceed its inherent strength limitations. All wood is substantially stronger along its grain structure than across its grain structure, although some species have an interlocking grain structure which affords much greater strength across the grain than parallel grain structured species. This additional cross-grained structure is not, however, sufficiently strong to afford sufficient structural strength in most any application. Obviously, the smaller the piece of wood of any species, the less structural strength it will have.

 

I've worked with wood for a long time and I know of only one method of strengthening wood that affords any real improvement over the wood's inherent structural properties and even that is rather limited. This method employs a particular cocktail of somewhat exotic solvents and epoxy resins which penetrate the surface of the wood. The solvents carry the epoxy resin into the wood and when the solvents evaporate and the epoxy resin which has soaked into the wood cures, the epoxy-soaked wood is essentially "plasticized." The cured epoxy employs the wood structure to form a matrix which is stronger than the wood or epoxy alone.

 

Some employ this mechanism by applying thinned epoxy resin to wood, the epoxy being thinned with solvents such as acetone, xylol. or toluene, but testing indicates that this simple approach is not as effective in penetrating the wood than commercially available proprietary "penetrating epoxies." The only proprietary penetrating epoxy formulation which has shown greatly enhanced abilities to penetrate (i.e. soak into) wood, both into end-grain and the face of wood surfaces is called Clear Penetrating Epoxy Sealer(tm) or CPES(tm). This product is manufactured by Smith and Company of Richmond, California. it is, however, repackaged and sold in some marine chandleries and hardware stores as "Rot Doctor." CPES has been around for probably fifty years now. It was originally developed for use in the preservation and restoration of decayed architectural embellishments such as  gingerbread" fret saw work, turnings, and carvings on Victorian wooden buildings. It was never designed nor advertised to be used to add strength to structural wood, but rather only to preserve trim. That said, it was quickly embraced by the wooden boatbuilding community because CPES was an excellent sealer beneath wood in the marine environment and a very good primer for bonding epoxy structural laminations in wooden boatbuilding. I've used gallons of the stuff over the years and can attest that, to the depth it soaks into a piece of wood, it adds strength, and particularly impact resistance, to the epoxy-treated wood. A full explanation of this product can be found at Smith and Company's website: http://www.smithandcompany.org/CPES/ (This product, which isn't cheap, but has a long shelf life, can be purchased in most marine chandleries and by mail order from the company's website. Other "penetrating epoxies" are also marketed, but I've been using the original Smith's product for almost 50 years and I don't know a commercial boatyard that has ever used any of the "off brands.")

 

Because there's a limit to everything, I can't say CPES is going to turn all your modeling wood into hard epoxy resin, but it will provide some additional strength to small pieces and "sticks." Whether it provides enough for your purposes will depend on the use to which the piece is put and you'll just have to experiment with it. (It's also an excellent primer for wooden hulls which will be placed in water, whether they be painted or resin-coated.) With respect to "sticks" like small scale yards and such, you might consider abandoning wood altogether and going to brass or copper. The construction standards for many museums and institutional model collections sometimes specify the use of noble metal spars in smaller diameters to minimize the chance of breakage otherwise present in thin wooden spars.

 

In conclusion, frankly, if your wood piece isn't strong enough, I'd think you'd be better off using a stronger species of wood or using brass for that part. It goes without saying that if your piece is "cross-grained," there's no point in expecting it to ever have much strength at all. 

Edited May 1, 2023 by Bob Cleek

[knightyo]

This was absolutely fantastic information, Bob.  Thank you!

 

Alan

[Dziadeczek]

I would simply replace the planks with, say, cherry - it is a bit "stronger" wood specie, similar color, although with slightly more pronounced grain. If you cut new planks on a table saw, you can carefully select better areas from your board and make your cuts along the grain and not across it or at an angle.

Cherry is a pretty common hardwood, so you should not have any troubles locating it in hardwood lumberyards in your area.

Edited May 2, 2023 by Dziadeczek

[allanyed]

Maybe go to European boxwood, (buxus sempervirens)?   Color is off, but I have found that is so tight grained that it is the strongest wood for small dimensioned stuff.  Janka hardess of 2.840 lbf for boxwood versus 1660 for Swiss pear and 1880 for castello (calycophyllum multiforum) 

Allan

 

[mikegr]

i have used 3 different products to strengthen some balsa fillets.

epoxy resin, fiberglass resin and wood hardener

 

Epoxy resin and fiberglass gave some limited positive results. The wood hardener, im not sure. All i know is that its smell was vey similar to a nitro thinner. Basic difference was the price hardener 10$ per liter while thinner cost 4$. So basically was a waste of money.

 

In short, i haven't come up with a solution than satisfies me 100%