Varnishing Help

Paul Wilson

Curious about Wooden Canoes
I hope someone can help with my varnishing problem. I read the article by Pam Weld on varnishing the interior of a canoe in the Wooden Canoe magazine and thought I would try it. I use Captains Varnish with T10 thinner. After the first coat of 60/40 varnish to thinner I notices that canvas has relaxed and had formed a few palm sized bubbles between the canvas and the wood. I thought it would tighten back up with time but after the 75/25 coat it was worse. I took the canoe out in the sun to see if that might help. It was turned upside down and allowed to bake for half a day. When I turned it over I noticed that along all plank seams the same relaxed canvas folds had appeared. To date it has not returned to its original tightness.

Has anyone experienced this and is there a solution short of recanvasing the canoe?

Since no one else has chimed in yet, I'll tell you that I've never had this problem, but I've always varnished (at least 3-4 coats) before canvasing and filling. If I've not put on al the coats of varnish before canvasing, the final coats go on after all else is done- canoe is filled, cured, sanded, fully painted, and any final trim work done. So my technique is different from that which you used, and I haven't had this problem.

Unfortunately, I'd be surprised if the canvas will re-tighted over time, particularly because any sagging or loosening of the weave (whatever you want to call it) in response to the thinned varnish will be locked in place as the varnish cures.

Wish I had a better prediction. We can hope I'm wrong- certainly could be since I've not experienced this problem.

Best of luck,
Varnishing interior


Thanks for the reply, this is the same advise given to me by Pam Wedd. I guess I have been lucky so far varnishing the interior after canvassing. Since this is a new idea for me I have another question.
Do you tape the planking seams before varnishing?
Do you varnish the exterior as well as the interior?

No, I don't tape seams. If varnish bleeds through, so be it. If it leaves lumpy runs on the outside of the planking, I use a scraper to smooth things out so no lumps show in the canvas. On the outside, I apply either boiled linseed oil cut with thinner or thinned varnish, after the interior has been well varnished. There is almost never any bleed-through from outside to inside along planking lines, but I always check and clean off any possible bleed-through with a mineral-spirits-soaked rag.

As an aside, there has been some discussion on these forums about possible harmful effects of linseed oil, and there's a widely used phrase (of little value, in my opinion) that "linseed oil is bug food" (search woodworking forums and you'll find someone saying this). The idea is that linseed oil can promote thr growth of bacteria and/or fungi, and this could be harmful to the wood and/or canvas in a wood-canvas canoe. Linseed oil is widely used, and has been used on most of our old canoes, many of which are now 50-100 years old or older. So while I believe linseed oil is fine and there is no proof otherwise, some believe it is best avoided. In either case, on an old canoe, it seems a good idea to get some sort of penetrating oil-based material on a dry exterior (but it must be in a form that cures and doesn't remain tacky).

Now, given all THAT, you might want to tax your neurons by reading discussions of blisters forming in the exterior paint- lots of talk about this on these forums. There are many possible causes and many possible solutions, but there is no consensus. Because I work on so mcuh other than canoes, a given restoration project usually has plenty of time for each stage to cure (varnish, oil, filler, etc.), and I've only seen blistering once. It was minor, was not along planking seams, and disappeared upon drying, never to return. I am convinced that it was because this canoe was recently painted, and then the owner left it sitting right-side-up on wet ground with rain falling on the interior for a couple of days during an outing.

I hope all of this is not more than you really wanted to hear. Search these forums for more information on these topics, try what feels right, and always remember- if you can restore it, you can fix any problems that may occur along the way. The short-term pain of having to go back to a previous step will pale with respect to the long-term gain of a job well done.

Varnishing interior


Thanks alot for your information. It has been very useful. You are correct when you say the effort is worth it so that the product is well done.
As an aside, there has been some discussion on these forums about possible harmful effects of linseed oil, and there's a widely used phrase (of little value, in my opinion) that "linseed oil is bug food" (SNIP) So while I believe linseed oil is fine and there is no proof otherwise, some believe it is best avoided. In either case, on an old canoe, it seems a good idea to get some sort of penetrating oil-based material on a dry exterior

Since Michael has taken me to task on a previous post I made, I will back it up with something a little more substantial than a generic "woodworking forum". From the Wood Handbook of the Forest Products Laboratory, I quote:

" Drying oils, such as linseed and tung, are sometimes used by themselves as natural finishes. Such oils are not recommended for exterior use unless they are formulated with a mildewcide. These oils are natural products and therefore provide food for mildew. When drying oils are used on highly colored woods such as redwood or the cedars, they tend to increase problems with mildew." (all boldface is my emphasis)

Further, one argument for applying linseed oil to the hull exterior is to prevent absorption of water. Again from the Wood Handbook, the moisture-excluding effectiveness of straight linseed oil on ponderosa pine, after three coats, was 33% the first day, 2% after 7 days, and 0% after 14 days.

As discouraged as I may be about our current federal adminstration, the folks at the FPL are pretty good and not to be discounted lightly. If you still disagree, show us some hard evidence...

(The part that gets left out when comparing to how the "old timers" did it, is that they used lots of white lead, both in the canvas filler and in the paint. Very few are left who are still willing to do that...but in my opinion, that is probably the key).
Count me into the crowd that won't use linseed oil on anything. I just won't put up with an oil finish that may stay sticky for a month and which often eventually turns black. I can't for the life of me understand why more wooden canoe people aren't using Deks Olje #1 as a penetrating oil and sealer (it also makes an awfully nice, matte oiled finish). It's dry to the touch in a day or two, never turns black, soaks in like crazy and is extremely easy to apply.

Straddling the boat/canoe market, I am quite aware that canoeists very often tend to be a lot more, shall we say "frugal" than the typical sailor or powerboater, so maybe most aren't willing to lay out the cash for a ready-made oil sealer that really works. Personally, I would never put the kind of labor that a good restoration takes into a canoe and then get linseed oil anywhere near it.

To me the most disturbing piece of info here is the note combining LO and tung oil from the FP Lab.

I've been using tung oil on the insides (with the mildewside packet) BUT, will it turn dark also like LO???

I can give anecdotal evidence about linseed oil on white ceder. In this case it's our cabin. I sided it with t+g white ceder dipped or sprayed (it vairied with batch) with a linseed oil based water repellant/wood protectent. After 5 years the black was starting on the bottom areas. Now, at 10 years, the black has progressed 2/3 up the sides. The material supposedly included a mildewcide. Note that this might be an extream application as it's wet/damp often.

Hi Dan,

Don't confuse mildewing with darkening. It appears both linseed and tung oils may be susceptible to mildew.

Linseed oil also tends to darken with time, whereas tung oil does not. If your oil is under varnish, it shouldn't be susceptible to mildew. I would expect that some of those old canoes with old varnish that borders on black had varnish that used linseed oil as the base.
Thanks Dan,

I feel a bit better now. I had "assumed" (ya I know) that the LO darkening reported was mildewing, but if it's something different, I should be OK.


just a point of clarification. Was the canvas filled and painted prior to varnishing also, or was it just stretched?

ie, did the evaporation of the varnish solvents interact with the raw canvas OR with the filler/paint?

Dan, please don’t misunderstand me- I wasn’t taking you to task, nor was I taking anyone else in particular to task over linseed oil. In fact, long before any previous discussion of linseed oil on these forums, I had read many reiterations of the phrase “linseed oil is bug food”. And while I agree that that the Wood Handbook from FPL is an outstanding resource, a quote from it without supporting data does not constitute “hard evidence.” To go a bit further, the statement that “These oils are natural products and therefore provide food for mildew” is a gross oversimplification. Just because something is a natural product does not necessarily mean that it supports mildew growth; in the larger scheme of things, very few natural products support mildew growth (and many natural products are in fact anti-fungal). Further, cedar is loaded with oils, as are all larger biological materials (every cell of every living organism). Oil is so abundant in cedar wood that it is expressed and used for many purposes- I have several liters on the shelf right now.

But this strays from the point. I agree fully that hard evidence on the merits (and demerits) of linseed oil is lacking. Yes linseed oil can support growth of mildew, particularly in its raw form. Pyrolized tryptophan has been widely discussed as a possible potent carcinogen, and is produced in copious amounts by grilling foods containing the amino acid tryptophan. Even so, grilling is extremely popular, and as far as we know there is not enough evidence to warrant banning barbeque grills (or even putting warning labels on them). My point re boiled linseed oil was meant to be that it may be useful to use something to restore some flexibility to dry, brittle old wood. As an aside, I took the general woodworking public to task over the use of the oversimplified and unsupported use of the phrase “linseed oil is bug food.”

I find the issue interesting, however, and am currently conducting an experiment to satisfy my own curiosity about the fungus-promoting capacity of boiled linseed oil on wood (particularly cedar), and the effects of such fungus on wood. The other concern, of course, is the potential effect of fungus on canvas that is tightly associated with linseed oil-treated wood. Many people use untreated canvas, filled with white-lead-based filler. If the white lead penetrates the canvas and is effective in protecting the canvas, then it may not matter if you slather peanut butter on the outside of the hull. Mildewcide-treated canvas and no-lead-based filler? I don’t know yet, but hopefully will soon. However this all turns out, it is my plan to write up the results as “hard evidence” for publication in Wooden Canoe.

Oh- almost forgot- the issue of oil decreasing water absorption by wood is something I have never considered. I have seen many old canoes sitting on the ground in the elements for years (even decades!) with the cedar still intact. Just for fun, I took a hunk from one and cleaned, bleached, and re-varnished it- looked wonderful! So even though the Wood Handbook quotes the lifetime of raw linseed oil as a water repellant at 3 days (and this is RAW linseed oil), I’m not sure this is an issue worthy of consideration.

Bottom line- I always appreciate your insight, Dan, and didn’t mean for it to seem that I was attempting to take you to task. That was not my intention. Personally, I have always treated the outside only of to-be-canvas-covered hulls with something to restore elasticity. I have used either thinned varnish (many formulations of which contain linseed or tung oil, by the way!) or boiled linseed oil mixed with Cuprinol (though the two are not readily miscible and I am unsure how well the Cuprinol penetrates under these conditions). So I just really want to know the answer to an interesting and important question- is boiled linseed oil really “bug food” in any significant way? Is this important in the capacity that some of us use it?