Accelerating polyurethane dry time


Dyami Plotke

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I'm rush to finish a wedding present be Thursday (tomorrow) morning. I'm finishing it in my air conditioned shop on Long Island. In the shop, its in the low 70's. Outside it's been in the high 80's and low 90's and very humid.

I put a wash coat of 1/2 lb shellac on Monday night, and it dried fine. Last night I rag applied a 50/50 polyurethane / mineral spirits blend.

Now, 10 hours later, it's still too tacky to sand.

 

I turned the AC up, thinking that will drop the humidity in the room.

Anything else I can do add to the mix to increase speed time? I need to install 4 coats in the next 18 to 24 hours.

Thanks for the help.

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Yikes, I don't think four coats in 24 hours is gonna happen with poly, Dyami.  Even in the desert that's pushing it.  You said this piece was (hopefully) going to be decorative...you might go back to shellac after the coat of poly dries.  You could certainly get four coats of shellac done in 24 hours.

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Yikes, I don't think four coats in 24 hours is gonna happen with poly, Dyami.  Even in the desert that's pushing it.  You said this piece was (hopefully) going to be decorative...you might go back to shellac after the coat of poly dries.  You could certainly get four coats of shellac done in 24 hours.

That's how I would go, Eric. Unfortunately I don't think shellac has enough body to hold the suspended grit in the final cost. :-(
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minwax makes a fast dry poly.   That is probably your best bet for subsequent coats.   But 4 coats in 24 hours?  you are more likely to find a pink unicorn in your back yard.   Japan dryer will speed things up, but you only need a tiny bit so do some research.    

Also thinning with mineral spirits does not make the poly dry faster.   In fact it might be working against you.  instead of 2 full strength coats or three 67% coats, you now need 4 50% coats to get the same amount of solids on your piece, each requiring a 12 hour+ re coat time.   thinning does help it go on a little better but 50% is probably overkill, especially if you are in a hurry to build a finish.  

turning up the AC won't help.  You want to lower the relative humidity.   Given the same amount of moisture in the air, hot air will have lower humidity.  raising the room temperature will be more effective.   if you cool the room too much the ambient air can not accept any more moisture, which will prevent your poly from curing.   if you heat the room, the ambient air can take on more moisture which will speed up curing.  

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minwax makes a fast dry poly.   That is probably your best bet for subsequent coats.   But 4 coats in 24 hours?  you are more likely to find a pink unicorn in your back yard.   Japan dryer will speed things up, but you only need a tiny bit so do some research.    

Also thinning with mineral spirits does not make the poly dry faster.   In fact it might be working against you.  instead of 2 full strength coats or three 67% coats, you now need 4 50% coats to get the same amount of solids on your piece, each requiring a 12 hour+ re coat time.   thinning does help it go on a little better but 50% is probably overkill, especially if you are in a hurry to build a finish.  

turning up the AC won't help.  You want to lower the relative humidity.   Given the same amount of moisture in the air, hot air will have lower humidity.  raising the room temperature will be more effective.   if you cool the room too much the ambient air can not accept any more moisture, which will prevent your poly from curing.   if you heat the room, the ambient air can take on more moisture which will speed up curing.  

Thanks, Mike. The poly is cut so I can rag it on, not to speed drying. The AC is on because it dehumidifies as it runs. The outside relative humidity is about 90%. Even with the AC on, it's not very cold, do I think the dryer air will counteract the colder.
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There are several solutions here...

Drive to local Hallmark store and get card apologizing for late gift. Take photo of said project. Enclose card & photo with a check for $500. I’m sure your procrastination will be overlooked. :)

 

Sand-back and apply four coats of shellac -- my personal choice...

 

Drive to BORG for the following:

http://www.amazon.com/Klean-Strip-PJD40-Japan-Drier-1-Pint/dp/B00166N1WG/ref=sr_1_1?ie=UTF8&qid=1441812722&sr=8-1&keywords=japan+dryer

http://www.amazon.com/Vornado-Whole-Vortex-Heater-Black/dp/B00MB4BQ0G/ref=sr_1_1?ie=UTF8&qid=1441812752&sr=8-1&keywords=vornado+heater

http://www.amazon.com/Master-Lock-S9900-Polypropylene-Caution/dp/B004LGSW0Y/ref=sr_1_2?ie=UTF8&qid=1441812770&sr=8-2&keywords=wet+paint+sign

The heater/fan will get the existing dry in a few hours. 10%  JD will dry typical oil/poly/resin mix in maybe 3 hrs with thin coats.  I would only apply one or two more thin coats with JD -- you would risk cracking. The sign is just in case... :)

 

Take your pick... and Good Luck...

 

 

 

 

Edited by hhh
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If you decide the heat + JD option, a few operational thoughts...

 

5% JD will provide a more controlled acceleration. 10% is what I’d term aggressive -- it's about as far as I'd push it. I’d avoid successive coats of aggressive drying to prevent cracking... Note: I've cracked finishes at 10%JD + too much heat...

 

Same goes for heat... I’ve got several of those Vornado fans... I frequently employ two or three surrounding the project all set on low. It’s better to move more warm air, than less hot air. I’ve found optimal drying around the low 80s... The more you raise the temp, the more aggressive you become and the more you risk cracking...

 

My most aggressive drying schedule: 5% JD, hang the project from the ceiling to dry and surround it with several Vornado fans set on low... I find I can consistently dry arm-r-seal in about 3-4 hrs... The more coats you add this way, the more risk you assume... I’ve done four coats in a day, but that’s crossing all fingers&toes...

 

NOTE: YMMV -- accelerating drying can lead to cracked finishes... I don't think I've ever cracked one or two coats, and believe that all my cracking has occurred in coats four or five. So it's sort of like the Holy Hand Grenade of Antioch* -- three shall be the number of the count... Five is right-out... :)

 

Note2: once you mix the JD+finish you must use it right away and toss what's left over. Don't be tempted to cover any remaining since you'll need more in a couple hours -- doesn't work that way... Mix a fresh batch with each coat... Yes, it's wasteful -- but it's the only way to make multi-coats work... And apply thin coats...

 

 

 

 

 

*Then did he raise on high the Holy Hand Grenade of Antioch, saying, "Bless this, O Lord, that with it thou mayst blow thine enemies to tiny bits, in thy mercy." And the people did rejoice and did feast upon the lambs and toads and tree-sloths and fruit-bats and orangutans and breakfast cereals ... Now did the Lord say, "First thou pullest the Holy Pin. Then thou must count to three. Three shall be the number of the counting and the number of the counting shall be three. Four shalt thou not count, neither shalt thou count two, excepting that thou then proceedeth to three. Five is right out. Once the number three, being the number of the counting, be reached, then lobbest thou the Holy Hand Grenade in the direction of thine foe, who, being naughty in my sight, shall snuff it."

 

https://www.youtube.com/watch?v=xOrgLj9lOwk

 

 

Edited by hhh
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  • 5 years later...

So a lot of y’all have the two main groups of finishes confused, and are offering advice for one vs the other, you have reactive finishes and evaporative finishes, reactive being urethanes or polyurethanes where a physical chemical reaction has to take place for the finish to harden and unfortunately unless you used a 2 part urethan where you pick the speed of the activator which cures it, in a can it has been picked for you so some heat will slightly accelerate it but not by much, then you have evaporative finishes like shellac lacquers where a resin or a coating is dissolved in a solvent and once that solvent is gone you’re left with a nice dry hard coating, that’s where low humidity, high air volume movement and slight heat will dramatically increase the curing time( so with all finishes you want to apply in a low humidity environment, air movement only cures evaporative finishes faster, with reactive finishes you’re just making the smell go away it doesn’t effect the chemical reactions taking place 

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@Aquatic123, welcome to the forum. I think you might be overlooking the fact that even "reactive" polyurathane finishes are carried in a solvent that must evaporate before the reaction can really take hold. Most of the comments revolve around how to accelerate the evaporation. Unfortunately, most of the original participants in this thread have been no-shows for some time, so I doubt there will be much of a rebuttal...

Once again, welcome, and thanks for chiming in!

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