The time to strip your canoe of old varnish is before you recanvas. The chemical strippers necessary for the job will seep between the planking and affect the filler and paint on the canvas if it is done either with existing canvas on the canoe or after the canoe has been recarnvassed. If the varnish is old and in poor shape, but you aren’t planning on recanvassing, then the best you can do is to sand and scrape thoroughly before varnishing.
Stripping old varnish from the inside of a canoe is a messy job best done outdoors. I use a chemical stripper called F400 that I get from Royal City Paints in Brampton, 905.791.6834. It is a liquid stripper containing methylene chloride and who knows what other deadly chemicals. I find the liquid to work better than the semi-paste. Depending on how heavy your varnish is, it will take at least two gallons. It works best at temperatures between 10-15° C , (50-6° F), and on an overcast day or in the shade. A hot sunny day makes it very hard to keep the varnish wet with stripper and you will either have a poor result or use much more stripper than is usual. I finish the process with pressure washing the inside and this stripper is compatible with this method. If you use a different stripper it will probably be a good idea to do some tests first. It could be that because this is not a semipaste, it is more compatible with the water wash – not sure but my guess it will work regardless.
I like to cover up with coveralls, hat, goggles, and latex gloves under a pair of heavy rubber gloves. Be aware that the stripper will eat plastic so watch out for your glasses. I use an old pair of “cheaters” and not my expensive prescription glasses for this job. They say that a regular charcoal filter mask doesn’t keep out the methylene chloride - I used a special filter that I put on my regular mask that says it take out the chloride, and it did seem to work. Lately I have been unable to get this special filter, and so now use the regular charcoal filters. They seem to work okay, and are definitely better than nothing if you can’t find the proper filter. There are also “safe strippers” that are less noxious and you might want to try those. They just might take a bit longer to work.
Set the canoe up on sawhorses in the shade. I haven’t found the stripper to affect grass but then I choose a spot out of the way in the long un-mown area behind the shop. I find it best to leave on the old canvas as it catches and keeps the stripper in the canoe where you want it to do its job. Pour the stripper into a 19 oz can and slop it on with a pure bristle brush. Be generous and work quickly down the length of the canoe. It helps here to have a friend to share the job. I go right back again and slop on a second coat and then cover the whole canoe with a sheet of plastic held in place with spring clamps. I leave it for about a half hour and then go back, peeling back the plastic part way down the canoe and applying more stripper. The trick is to keep the inside pretty wet with the stripper - if it dries right out then it just re-adheres and you are starting from scratch again. This time around I apply the stripper with an old plastic kitchen scrubbie brush and gently work the now loosened varnish down to the floor of the canoe. You can reuse the liquid that puddles in the bottom even though it looks all dark and vile - as long as it is wet it will still strip. I gradually work my way down the canoe, lifting back plastic and replacing it as I go. By this time the varnish will be pretty much off and be a gucky mess in the bottom. I take an old spoon and plastic squeegee now and pick up as best I can, the majority of goop from the bottom and place it in an old paint can. Finally I go over the whole canoe again with paintbrush and new stripper and make sure every part is moistened.
I always finish up by washing the interior with TSP. I am not sure if it does anything but it makes me feel better and makes sure there is nothing left to interfere with our brand new varnish. You will have to check into the stripper that you use, as you might find it necessary to rinse with alcohol (methyl alcohol) or other thinners first. The canoe will be soaked at this point and will need several days to a week to dry out sufficiently before work can begin.
Before varnishing I thoroughly sand the interior with 120 grit on the ribs and either 3M purple pad or 220 on the planking. It is next to impossible to sand with the grain on the planking so I go up and down, across the grain but do so very carefully so as to leave very little scratching. If the stripping job has been really good sometimes I won’t touch the planking.
Once all the woodwork is done, now is the time to stain any new wood to match the old. And it is often a good idea to give the interior a coat of oil mix (see below for recipe) to give back some flexibility to old brittle wood. I like to let it dry now for 2 or 3 days before proceeding with the varnishing.
If you haven’t oiled the interior, then the first varnish coat should be cut 50% with mineral spirits to aid in its absorption into the wood. If you did oil the interior then go right to coat #2. After 24 hours I knock off the raised grain of the ribs with 180, and again usually don’t touch the planking. Coat 2 is usually cut by 20 to 25%. This is followed by sanding the ribs with 220 and the planking with the purple pad. Coat 3 is cut by 5%. I usually find that 4 coats is enough on an interior so the sanding now is 220 on ribs and 320 on planking. This is the time to be diligent and sand out any sags, runs or dust before the final coat of varnish.
I use Epifanes Varnish and find it builds up quickly and has incredible gloss. Petite or Woolsey Hi Build and Z-spar is also good varnish and a little thinner and easier to apply. Its gloss doesn’t seem quite as good and it is a little more yellowing. I have found it helpful to hang a light bulb down into the middle of the canoe, and up off the floor of the canoe by an inch or two - it sets up an air current that moves the solvents out of the inside of the canoe and helps the varnish to kick. Another option is to place a small fan on the gunwales, pointing down into the interior to help move out the solvents and speed drying.
It is usually a good idea to do all varnish coats on the interior before canvassing and filling the canoe. Occasionally, depending on the filler used, varnish after recanvassing can affect the canvas where it seeps through the planking creating unsightly blemishes on the exterior painted surface.