Canvas Filler Chemistry


Inquiring Mind
I am now starting the "4 weeks curing period" suggested by many for the length of time canvas filler requires to “harden”. Now, is this a chemical reaction as suggested by this I found on the web, specifically with respect to "Japan drier"?:

"The prime ingredient is a heavy metal (like cobalt as someone mentioned) and it acts as a catalyst in the polymerization of the oil into a plastic. The polymerization will take place without it if exposed to air and given enough time. With the cobalt the process will happen in a few hours or so."

So I'm thinking "If all the reaction has already taken place, what happens during the next 3 weeks, 6 days and most of another day?"

There is no water, or such liquid/moisture that has to "dry" by evaporation, is there another type of outgassing going on? Is there a physical, chemical reason why I can't now start applying an undercoat, or primer, or first coat of paint to the hull?

I might be convinced by those of you with some experience that a 4-week period would allow the canvas filler to set up more than it has already and become more structurally strong, but would the application of paint now hinder this process?
There is no way your filler is curing in 6 to 8 hours. Just ask anyone who has painted there canoe prematurely only to later discover little oil-filled blisters all over their hull.

You can't rush it, and 4 weeks at 70 degrees is the rule of thumb. If you'd wanted to push it, you should have used a different filler...
Thanks for the info, but that didn't answer any of my questions!

You mentioned “bubbles of oil” but is this a normal thing the filler does, or is it a case of too much oil in the filler mixture, or too little “japan drier”, for example? If oil bubbling to the surface is part of the process then yes, I agree it's be best to wait for 4 weeks for this to stop. So then, what process is causing the oil to be forced out? Or not integrated into the rest of the filler?

If the surface had several coats of enamel on it, why wouldn't the bubbles of oil just escape through the inside of the canvas? Gravity?

It seems to me that in two of the formulas given on the site, we can compare the amounts (never mind the pounds, quarts, pints and ounces) of the one from Wooden Canoe #20 and #31, and they are:

Ingredient #20 Amount/#31 Amount

silica 5 / 4
linseed oil 1 / 1
turpentine 3 / 6
Japan drier 16 / 7
lead 2 / 4

If we assume both these mixtures were adequate, I'm wondering why only linseed oil and silica are reasonably similar amounts. What makes one mixture different by having twice the lead, twice the turpentine and less than half the Japan drier? If someone said to you "You can have the same stuff if you use one extra measure of silica, twice the amount of Japan filler and half the lead, and half the turpentine.", does that not seem like a distinctly advantageous proposition?

So why then would two very different mixtures each take 4 weeks @ 70 degrees to cure?

Something else occurs to me: was the original use of lead in this chemistry to supply the heavy metal catalyst for the polymerization of the oils, and only incidentally did it's beneficial toxic effects (to prolong canvas life) become known and accepted?
Like I said, I ain't no chemist, so I won't even pretend to answer most of those questions. Maybe someone else is/can. However, what you see when the filler is insufficiently cured before painting is that on first use, (or when the dew settles, etc.), the unpolymerized oil migrates out of the canvas. At least some if it gets trapped by the paint layer, and a blister results.

If you get blisters, and on popping them they are full of oil, it has to be the linseed oil in the filler (or from treating the planking prior to canvassing). That's the only place the oil can come from. Every year at assembly, we gather to goggle at a few canoes that blister. In many cases, we find the filling and painting was rushed to get the canoe finished in time to show at assembly.

There are other causes of blistering as well. It is a subject that has come up often here (many times before the big crash a couple years ago). I believe insufficient mechanical bond between filler and paint (e.g. wetsanding with too fine grit) to be a major culprit.
I ain't no Chemist either but many time I have pretended to be a Blister manufacturer.
In my humble storage of knowledge I would say that the japan drier, while it speeds the curing process it is limited in how effective that it can be. The canoe filler is put on much, much thicker than any paint manufacturer ever dreamed their paint would be used. The oils and thinners used to make the filler to be smoothed must evaporate out of the filler before being trapped by the finish paint. The top layer of the filler will harden very fast and this just slows down the ability of the oils and thinner deep in the filler to evaporate. Adding even more japan Drier may seem like a good idea but that will cause the filler harden and age prematurely and cause the filler to crack sooner than normal.
If you want a faster process, use something else!
Bad case of the blisters

The blister issue really bugs the scientist in me. I keep experimenting.

I had the Prospector on the recent Moose River trip. It was a wet trip so the interior got a thorough soaking with rain and there was no chance of drying out much during the four days. The paint job eventually blistered heavily.

The canoe was oiled prior to canvas and allowed to dry.

I used lead free, oil based filler.

It dried for probably three months or more before sanding and painting - temperatures varied from below freezing to probably 80 F.

It has only paint on the filler - no primer etc. 4 coats marine enamel sanding between coats.

Sanding dry with 220 grit (finest grit used).

Based on my observations, I don't think oil is involved in this heavy blistering. Only water migrating from the interior of the canoe, through the canvas and filler and lifting the paint.

I give the canoe a few days to dry and the blisters vanish.

I probably tend to put heavy paint coats on - this might be part of the issue. I'm slowly changing my habits. I'm thinning the paint more lately too.

Otherwise, I'm at a loss. I wonder some about topside enamels performance when things get really wet. FWIW, I do have other boats finished by others that have similar blisters on occasion.

Steve Lapey's boat on this trip got just as wet - traditional leaded filler, and porch and deck enamel. No blisters on the trip.

Regarding filler drying - I have a canoe drying now. I use the fingernail test. It has been drying for 4 weeks now, and we've had cold and damp (as in DELUGE) weather. It still flunks the fingernail test. I also use sense of smell. I can still smell the canoe drying. When I open the garage and can no longer smell the filler - She's done!