sunlight curing for poly? (2024)

[OP]Member

I know first hand the effects of long term exposure of sunlight to polyurethane finishes, so I don't need much advice there, but another question arises.

After application, it takes about 24 hours for the poly to "dry to the touch", but actually takes about 1-2 weeks to harden (or at least so it seems - I don't have a way to make any more scientific measurement of this.) It is during this period of 1 to 4 days after application that I am wondering if one might accelerate the hardening or "curing" of the finish by direct exposure to sunlight for a few hours each day. Of course, no exposure to moisture would be allowed.

Every time I have wondered, I didn't want to risk the project to try it out, so I was wondering if anyone else has any experience trying to accelerate the hardening either with sunlight or some other method?

TIA,

Member

Gary
If you had a way to keep the piece cool and not get hot it might work. But if it's a dark color it's going to heat up before any curing can take place. Which will soften the finish before it hardens it.

Contributor

AFAIK, none of the finishes used outside a few specialized factories cures by UV light. I don't think sunlight will speed up the curing of any finish typically available the consumer/wood working pro. Heat may speed up the curing process, but that would be a dangerous game to play.

John

Contributor

Polyurethane finishes are HIGHLY susceptible to UV damage. Putting it in the sun would be a BAD idea.

Hmm... why do they make poly exterior varnishes?... they get to sell it every year; because it gets chalky in the UV environment.

Once it's dry to the touch; a fan blowing air across the surface will help speed the curing. (oxygen is what facilitates the curing) It will take ~ 30 days.

Member

Well, I may have been doing damage to my projects all these years but I used to place projects sprayed with polyurethane out in the sun for several hours and it definitely speeded up the drying time. I'm sure it was the heat rather than the UV exposure that sped up the process. Some of these projects are now over 30 years old and they don't seem to have suffered any from such a brief exposure. I no longer use polyurethane so I haven't done that in a while. Lacquer cures fast enough without any time in the sun. More recently, I have placed signs sprayed with rattle can enamel out in the sun and it sped up drying time of that material without any apparent ill effects. I hope that wasn't a mistake.

Member

I agree with Art. Here in AZ I do most of my spraying outside, and I leave it outside for a while to dry. I am convinced that the sun and heat greatly accelerates the curing time. I mostly want to get it dry enough to handle as soon as possible, and that happens faster in the sun, but I will bring the work inside at the end of the day and leave it inside after that. I have no data to prove it, just lots of experience. And like Art, I have see no negative effects over time. This also applies to my experience with lacquer.

Member

>>>> none of the finishes used outside a few specialized factories cures by UV light.

Correct. Oil based finishes like varnish or poly varnish actually "cure" by oxidation. In other words it's the absorption of oxygen by the varnish that causes it to harden.

Contributor

I think the camp that says air flow and the other that says heat are both right. Varnishes cure by oxidation, shellac and lacquer by evaporation. Air flow will accelerate both those processes, as will an increase in temperature (reaction rates generally double for every 10 deg C increase). UV light, however, nope, it's the heat in sunlight that's doing it, as well as the high probability that there always is some air movement outside.

John

[OP]Member

Well, thanks for all the responses. Definitely some food for thought. IR from the sun might just help a bit during the period, as well as a slight breeze once its dry to the touch. Perhaps I'll give it a try.

Contributor

Thanks for bringing this up. I was wondering the same thing.

Moderator

I've made this mistake before.

I no longer put things in the sun. It's very difficult to control the heat. Heating after initial drying has begun is a dangerous thing: Trapped solvent can't make it's way through the surface slowly. It can cause bubbles or cracking as the solvent pushes through.

When spraying this is less of an issue because the coats are thinner. But if I were brushing or even wiping on a finish, I personally would not put it in the sun.

I wouldn't even put a fan on the piece. Near the piece yes, but only to keep oxygen in the air flowing - not blowing on the piece itself.

Member

Off hand I can't think of anything you can put in the sun to cure that is beneficial. Even concrete sitting in the sun should be wet from time to time while it's curing. I saw a guy onetime put a fresh formica countertop in the sun and the laminate bubbled up to where it was only stuck around the edges. I've seen varnishes and lacquers develop bubbles in it from sunlight. It doesn't take long at all before it can generate enough heat under the surface to build fumes underneath.

Member

I'm having a hard time relating the curing of concrete with the curing of polyurethane. What is the connection? As I stated back when this thread started in 2014, I have used direct sunshine to hasten the curing of polyurethane on many occasions and I have never seen any adverse effects. If there was a significant risk involved in exposing the material to direct sunlight, it seems to me the manufacturer would have said so on the label.

Originally Posted by Edward Dyas

Off hand I can't think of anything you can put in the sun to cure that is beneficial. Even concrete sitting in the sun should be wet from time to time while it's curing. I saw a guy onetime put a fresh formica countertop in the sun and the laminate bubbled up to where it was only stuck around the edges. I've seen varnishes and lacquers develop bubbles in it from sunlight. It doesn't take long at all before it can generate enough heat under the surface to build fumes underneath.

Member

Originally Posted by Art Mann

I'm having a hard time relating the curing of concrete with the curing of polyurethane. What is the connection? As I stated back when this thread started in 2014, I have used direct sunshine to hasten the curing of polyurethane on many occasions and I have never seen any adverse effects. If there was a significant risk involved in exposing the material to direct sunlight, it seems to me the manufacturer would have said so on the label.

My point was that most everything you work with is negatively affected by direct sunlight. Sometimes you have to finish in the direct sun but the wood and finish is usually the same temperature and very gradually rises as the day progresses. Still if you rush it and put too many coats on too fast the finish can bubble.