Dry Spar Varnish in Oven? Or other methods to speed drying

Question

We are making some wooden craft items as Christmas gifts. They are small items (wood disks about 3/4" thick and 4" in diameter) that we've put two coats of spar varnish on.

The second coat of varnish (applied about 72 hours ago) seems to still be a bit soft / tacky (especially when two items are stacked / touching each other), and I'd like them to be fully cured and hard before shipping. (I will wrap them individually in bubble wrap or similar when packed for shipping).

They have been drying in relatively low humidity (Colorado), inside at approximately 70F.

I'd like to speed the full-cure drying time of these items.

Is it safe to "bake" these in an oven? Our oven's lowest temperature is 170 F, which seems like it could be a bit hot.

Or, could I place an electric space heater in the same vicinity as the items?

What can I do to speed the drying of these items?

Answer 1

Is it safe to "bake" these in an oven?

Not really, no. You wouldn't be the first person to try this and you might get away with it as some have in the past, but if it doesn't work as hoped it could lead to the only option being a complete strip and re-finish.

There is a potential health risk involved in doing this in the oven you cook in, but to focus on the woodwork you can't reliably do this in an oven with a minimum temperature of 170°F (77°C).

Warm is good, but that's a bit hot for safety. And in addition you can't be sure how hot the oven will get because most ovens don't hold at the set temperature the way we think — "hold" in this case being a misnomer since ovens don't do that at all. Instead they cycle on and off to approximate the set temperature. Additionally the thermostats of ovens are infamous among cooks for being off (reportedly by as much as 50°F!) which with wood is all kinds of bad as the water in it will turn to steam and bubble through your finish. I've seen it happen and it ain't pretty!

Or, could I place an electric space heater in the same vicinity as the items?

Much better option.

What can I do to speed the drying of these items?

• Increase air flow (to begin with to take away the evaporative portion of the varnish, after that oxygen is needed for the curing process to progress).

• Increase temperature (both reactions proceed more quickly at higher temperatures).

• Decrease humidity (water vapour can retard varnish drying and curing).

Obviously that last one shouldn't be an issue for you.

As for the first one, in reality normal rates of air exchange for typical domestic interiors are enough for reliable varnish drying so the main thing to focus on is increasing the temperature. There is such a massive difference in varnish drying with only a 10° change in temperature just giving it a little boost should make a world of difference.

(I will wrap them individually in bubble wrap or similar when packed for shipping).

If there is the least tackiness still remaining it would be advisable not to put the bubble wrap directly in contact with the varnish. Instead cover the pieces in a smooth plastic (black refuse bags can work well) and then wrap in bubble wrap.

For next time
I can highly recommend thinning your varnish with between 25 and 50% additional spirits, turning it into 'wiping varnish'. This can be wiped on as the name suggests, or applied by brush or roller if preferred. Regardless of the application tool you then wipe away some or most of the excess. This builds a shiny finish much more slowly, but each coat is very much thinner so dries far more quickly and reliably than when the varnish is used at full strength.

Answer 2

I haven't tried any of these methods myself but the following is offered up from a quick google search:

1. Reduce the humidity in the environment of the drying varnish by running a dehumidifier. Pointing a fan at the drying varnish may also help.

2. Wipe the surface of the varnish lightly with a rag moistened with turpentine or mineral spirits. Don't rub the varnish, just wipe it lightly enough to remove the tackiness from the surface. Give the varnish another day or two under the breeze of a fan to cure.

3. Brush or spray a light coat of clear shellac on the surface if the varnish is still tacky after wiping it with a solvent. Shellac hardens by evaporation and doesn't have to cure. Use shellac only if the varnish has almost hardened but is still slightly tacky. Don't use it if the varnish is soft.

4. Strip the varnish if none of these strategies work. It's probably tacky because the wood wasn't properly cleaned before varnishing. Wash the wood with a wax-removing detergent after you've stripped the old varnish, sand it and apply a fresh coat.

Varnish will dry in two phases. First the solvent, if any, must evaporate. Then the product must cure. The curing is a chemical reaction between the remaining materials of the varnish and/or the air. Option 1 above helps with speeding up evaporation but offers no help in speeding up the curing time; instead recommending to remove the uncured portion or cover it up. Both seem like poor options.

Chemical reactions can always be accelerated by increasing the temperature of the solution. Your instinct to apply heat is correct but don't put it in the oven.