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Painting in high heat/humidity


Buick35
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Yesterday I decided to paint my front fenders on my 35 Buick. I know it was probably a mistake but I got tired of waiting for better weather. I'm in Florida and the weather won t change much until mid October. Anyway it was about 80% humidity and 106 degree heat index.I'm using single stage acrylic urethane. It's been a day and a half and it feels like the paint is still soft in spots,hopefully it will finally cure. Also some checking of the paint in areas,I'll try to wet sand it and buff it out maby next weekend,I'll have to put another coat on some areas as well as some primer is showing through.Wish me luck,then all that's left to do is the running boards and putting it back together.Was I wrong not to wait for better weather? Thanks,Greg.

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Something is very wrong.

 

Did you use the correct proportion of activator to paint? If you used a reducer, was it 'slow'? In your 100+ degree heat you should not have needed any reducer at all.

 

The checking of the paint indicates you may have put the paint on too thick and didn't give the previous coat time to flash off. In general, you meed to put coats on 15-20 minutes apart (although you are working in high-heat conditions)

 

The fact that the paint is still soft in spots is very worrisome. Did you paint outside in the 106 degree heat and 80% humidity? That may be your problem right there -- the high humidity got into the paint. You may have to move inside where you can use de-humidifier and A/C just before painting to get conditions better.

 

Ask a couple of local auto paint shops about your situation. They may have an answer for you. 

 

Keep us informed.

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Two days but the guy at the paint store said with urethane it's easier if I didn't wait too long. I guess I'll wait longer on the other fender. I still have to re coat some areas but it's a learning process. It'll all buff out in the end. Greg.

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With the high humidity we are having right now in Florida and the high temperature it's taking the paint much longer to cure. I've been painting small parts in my Garage on my Triumph TR6 and what should take hours is taking days using spray cans. Another project is a wine cabinet I'm making out of cherry wood. About a month ago tried to spray the Minwax polyurethane coats. Sprayed the first coat, and it flashed on me with a foggy blush. Sanded it down, thinned it a bit with mineral spirits and it flashed again and that was spraying it at 8:00 in the morning.  I've given up until this 90 to 100% humidly leaves us. Without a booth with controlled atmosphere, it's tough this time of year.

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High humidity will slow solvent evaporation which affectively slows the curing but 80% humidity often isn’t the tipping point as most materials will still work at 85% under the right conditions and good air movement. If additional solvent was added to perhaps improve spray ability because it was hot the problem would only be worse.

The heat could also be the reason for the checking as the top part of the paint is drying and contracting faster leaving a crack like appearance. The heat can also cause bubbles in the coating called solvent entrapment. 

Depending on how thick the coating is you maybe able to sand it out but often you can’t. The recommended curing time is at least 7 days however it will seem hard much earlier.

Of course you should always measure the temperature and humidity at the place you’re working rather than using the forecast as your site can be significantly different from the forecast.

Edited by 26-25Buick (see edit history)
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