Treating inside of canvas

[Todd Bradshaw]

Before you do, try it on scrap canvas, let it dry completely and then test with some filler. The Scotchguard is most likely a fluorocarbon repellant, rather than a silicone compound but even so, one of its typical main functions is to prevent stuff from sticking well to cloth fibers. In fact, I'd do two tests, maybe 12" square, one sprayed, one plain. Then fill both, let it harden properly and then flex them a bit to see if there is and difference in the bond strength. The idea of what a mess you would have on your hands if the filler started peeling off of a new canoe should be enough motivation to conduct some tests.

 

[David Satter]

I only spray the inside of the canvas with a light coat. to protect from water migrating through the planking. It doesn't penetrate the canvas. I wouldn't treat the outside. Yes it could interfere with filler.

 

[Todd Bradshaw]

I'm not suggesting that you spray the outside. I'm suggesting that you test your inside-sprayed canvas, because if there is enough of the product on the inside of the cloth to actually do any good, then there is very likely stuff meeting the filler, somewhere inside the weave. Do what you want, but when mixing different technologies, I think it tends to be safer and more responsible to do a bit of testing first, just to be sure.

 

[Todd Bradshaw]

Actually, if my approach was to prevent the canvas from picking up water, I'd use something better than Scotchguard if tests would show that it didn't inhibit getting a good filler bond. The product that I would try is 303 Fabric Guard, and I would spray a decent amount on the inside of the canvas, as partial coverage doesn't usually do much. Once dry, you can't tell the fabric has been treated until you put water on it - no shine, no waxy texture, no visible changes. This is a test I did several years ago on two chunks of Egyptian Cotton sailcloth, one plain piece and one sprayed with Fabric Guard. I just put them on the counter and dumped water on them. About ten minutes later I took this photo. The untreated piece had instantly been saturated and remained that way. The 303 piece was still doing quite well with no signs of water being picked up in the treated area. If this sort of treatment on the inside would not change the filler's adhesion, it might be worth doing. It's about $80 per gallon (Amazon, Home Depot, etc.) but might still be worth it. On cotton sails it certainly is as they tend to have lots of multi-layered areas that stay damp long after the rest seems dry, and those spots grow mildew and mold. I used the cheapest Harbor Freight HVLP spray gun I could find to apply it.

 

[David Satter]

I'll look into that, I think I have some I use it on my old sails on my sailboat.

 

[Todd Bradshaw]

I had a little bit left in a squirt bottle, so I cut a small chunk of Old Town canvas to try it. Their squirt bottle is a pain, but my intent was to try to spray an even layer with full coverage, but not go overboard trying to soak the canvas. After spraying the potential inside the surface was lightly wet. The opposite side was more like damp. I stuck it in front of a heat vent (heat off) and let the fan blow on it for about 1/2 hour. Then I applied the water and here are the results.

The only filler I have on hand is an unopened gallon can of Old Town filler from decades ago, so I'm not going to crack it open for a test, but at least I can say that the water repellency seems pretty impressive and if filler worked OK on the outside it might be worth testing and trying.

 

[shelldrake]

Great information Todd. Thanks. If anyone gets around to testing the compatibility with oil-based filler, please let us know the results.

 

[Rob Stevens]

Seems to me that waterproofing the inside surface of the canvas will promote rot by trapping water between the planking and filler/paint.
Do you have 20+ years to test this theory?
Zinc naphthenate works.

 

[shelldrake]

I would imagine that the rot potential would be reduced if the canvas did not absorb and hold water, along with the advantage of not gaining weight on an extended trip.

Speaking of water absorption and rot; if we were serious about avoiding water absorption, it would be a good idea to seal the inside surfaces of the ribs and planking prior to construction.

 

[Todd Bradshaw]

It has always seemed a bit crazy to me to make so little effort to seal the back side of planking and then leave it in direct contact for weeks at a time with a wet piece of cloth. It is just asking for trouble and in most other forms of wooden boatbuilding, it wouldn't be done. I attribute these things to the idea that wood/canvas canoes were never really intended to last 100 years. They can, and do with proper care, maintenance and protection from weather, but they were not really built or intended to do so or with that in mind. If there are small things we can do to improve the odds, I have no problem with that.

 

[Douglas Kestell]

I use Copper Coat made by Rustoleum - it’s copper napthenate on untreated canvas. I get it from a big box store in gallon cans. A gallon will treat a 18 ft canoe. I apply it with a pump sprayer. It dries fast (day or 2) but the stuff smells awful and it lingers for quite a while after it dries, but I think it works well. I think that it shrinks the canvas as it dries as well.

The Zinc naphthenate is less effective than the copper naphthenate but coupled with proper drying it should be more than effective enough.

I’ve considered using the zinc for the problem rot areas before varnish like the stems, inwales, cant rib tops and underside of the decks that never get enough air circulation.

 

[dhansen]

How about using some sort of fiberglass cloth and epoxy........................

 

[Todd Bradshaw]

That's one option, though it comes with its own strengths and potential liabilities, nothing is ever perfect from all angles. My 1972 keelless Old Town 16' Guide. It's wearing two layers of 6 oz. fiberglass, installed with WEST 105/205 epoxy. Weight came in a couple pounds lighter than it originally weighed with canvas. Other than one fresh coat of interior varnish about 1980, the varnish is original.

Do be aware though, that doing this correctly is absolutely not a good beginner's epoxy/fiberglassing project. The vast majority of fiberglassed wooden canoes end up being a real mess.

 

[Gil Cramer]

Amen!!