Wood/Oil/Varnish Confusion


Inquiring Mind
I noted several discussions on the topic and the associated obviously very knowledgeable replies, but I'm still not clear.

The canoe I'm working on has been dry as a bone for years and creaks and cracks and pops when you beathe on it.

Dan M suggested using thinned varnish on the inside, followed by less thinned coats. I assume this extra penetrating action will get varnish deeper into the wood and make it stiffer, but the already dry wood's pretty brittle to start with.

Now, I DO want to varnish first (that makes sense to me), but would a less-penetrating application be better from the point of view that after the interior is sealed, a good soaking of oil to the exterior would bring back some "springiness" to the wood? Do I even want that?

If I do, it seems to me that the exterior oil won't be penetrating too deeply as the varnish, however much thinned, will have pretty much sealed the wood to keep out anything, including the possibly beneficial oil.
Thinning varnish is a tried and true method of getting good penetration and good adhesion of varnish with respect to the wood. The reason for going with less thinned coats later (ultimately unthinned except to control brushing characteristics) is to build a significant (i.e. protective) layer of varnish on the wood surface. Remember, this prevents oxidation of the wood, decreases abrasive damage, and inhibits UV-induced damage, assuming you're using a quality UV-shielding varnish.

As for oiling the hull, oiling the inside can lead to darkening over time as the linseed oil slowly oxidizes. Applying it to the outside is a non-issue with respect to darkening on a canvas-coated hull. As for penetration, the exterior of your hull is likely quite dry, and many manufacturers left exterior surfaces of planking pretty rough (though some hulls are wonderfully smooth). Dryness and roughness will ensure that the oil penetrates very well, particularly if thinned, say 50/50 with mineral spirits. If you're at all concerned, take a weathered piece of cedar (old canoe planking would be great), apply thinned linseed oil to it, and later slice it open with a razor blade or box cutter- you should see excellent penetration.

Your worry about the interior varnish inhibiting penetration of oil from the outside is also a non-issue. Many varnishes (true varnishes) are oil-based; even if the chemistry of a varnish is not based upon natural oils, varnishes generally are formulated to provide some flexibility when cured. If this were not the case, surface coats would shatter when pressure is applied, either from bending or from impact. Thus, varnish and oil both add flexibility to the hull. While they don't "add moisture" as is often stated (moister = water), they do bind dry wood fibers together in a flexible manner. This is what you want.

I oiled the hull of this canoe with 50% boiled linseed oil and 50% mineral spirits with great results. Attached is a picture showing the 1/2 of the hull oiled and the other 1/2 not.


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In my earlier post, I questioned the use of linseed oil on canoe hulls. Linseed oil is mildew food. Mildew rots canvas. White lead kills mildew. White lead causes other problems, so most, but not all, builders no longer use white lead. Before slathering your hull with mildew food, you might consider how you are going to combat it in your canvas treatment and filler... I agree with Michael that some hulls can benefit from treatment with linseed oil, but it may be at the expense of your canvas' life expectancy.

As an alternative, you can treat the outside of the hull with thinned varnish as well.
That's nice to know. "The Wood & Canvas Canoe", page 169 - 170, tells you to " apply a very generous coat (slathered) of hot boiled linseed oil and clear Cuprinol", 3/4 oil to 1/4 Cuprinol. I used mineral spirits in place of Cuprinol.
I'm realativey new at recanvassing canoes, this is my first one. I'm all for doing correctly. Which is correct, use linseed oil or not?
Which is correct, use linseed oil or not?

Conventional wisdom has been to use it. Lots of folks do it, I used to (might even again), but clearly there is some room for debate about it. I think i've offered some valid reasons to consider not using it. Others have provided good reasons for using it. Continue the debate, then weight the pros and cons against each other.

Cuprinol was added to the linseed as a preservative - it provided the mildicidal action. Cuprinol is no longer made in a formulation that is compatible with oil. DAP used to make one that was, but it appears to be gone as well. Most of the water-based preservatives also have water-repellants (silicones) that will interfere with your finishes.

I see no reason you couldn't add mildicide to your oil - it comes in little packets from the paint department, and is compatible with oil and water based finishes. Nasty stuff, though...
Thanks for the information Dan. I Googled "linseed oil mildew food", sure enough,it is. It's too late for the canoe I just oiled. I'll approach the next one differently.
For what it's worth, for the outside of the canoe hull, I have been using Olympic Brand Clear Wood Preservative, it is mildew resistant, and has linseed oil in it, seems like a good combination. I typically up the linseed oil a bit, as I like the idea of the oil keeping the wood from drying out over time.

If you haven't varnished your canoe yet, just mix up a little more oil/thinner, and add the mildicide packet Dan talked about, (Though I'd probably only use maybe 1/2 the packet or less. They are sized for 1 gallon and I'd only mix 1-2 qts at most.) and add a bit more oil to your hull. Most "home centers" carry them in the paint depatment. They are about $1/each.

Mucho Thanks

Good info all 'round, boys!

I agree with Dan; continued debate is of considerable benefit because as Frank Zappa used to say "Without deviation from the norm, there is no progress." (I paraphrase).

I can still get oil-based Cuprinol here in the Great White North (as Pam mentioned a while back) so I will opt for that and I can't see any reason why not to include a half mildicide packet as well.

Thanks all.
More on linseed oil and mildew.

What about taking all those precautions; using mildicide packet, using treated canvas, and them only oiling the inside, not the outside of the planking. Pros? Cons? Splinter