Automotive Urethane Clear in 70% Humidity

[Len Woodruff]

Anyone successfully use 2 part automotive urethane clear in high 70% humidity?

[Ace-Garageguy]

Yes. On real cars and aircraft, where problems can be VERY expensive.

In the summer here, the humidity is routinely in the 80-90% range. When the humidity is high and temperatures are over about 90deg F, solvent popping can be an issue.

Use the right reducers and activators / hardeners for the temperatures you're working in and you should be OK...unless it's VERY hot and VERY humid.

READ the technical data sheets  (TDS) for any material you're using to understand the effects of temperature, and the alternative products and procedures to use.

[Len Woodruff]

Thanks Bill. I shot it yesterday. The 1st coat went on perfect but the second coat went on like I was shooting a flat color. That may have been because the pot life was shorter than I expected. But it was completely cured in less than 3 hours.

[Bob Ellis]

I can never clear coat in the Summer near Boston when the humidity is more than 70%.

The problem is also that the evaporation of the volatiles in Summer cause the humidity to condense as the gases expand and get cold against the hot, thus water is in or on your paint work. You need A/C, good weather or some additives to reduce the fast evaporation of the volatiles.

[MGL]

I grew up at my Dads body shop and learned to paint there with no paint both, spraying enamals, lacquers, and urathane paints. in the winter we would paint in the early afternoon when the shop was warmest and in the summer we would paint in the evening when the shop was coolest. This was southern MD where the humidity was regurlerly above 80%. If you see moisture condinsing in the evening, all that water came out of the air lowering the humidity. Good water traps will keep it out of the lines and an additional measure we used to take was to run a section of the coiled hose through a cooler full of ice in between the line coming the compressor and the line to the paint gun (we had air lines hard piped along the walls). The ice in the cooler would condense any remaining moisture out of the air and also keep the beer cold. We never had issues with moisture when we did this. As far as off gasing or evaportion causing problems with the paint after its sprayed I think matching the temp range of your reducers or using additives to slow it down will help but I haven't painted a 1/1 car in over 10 years so I'm not sure what additives are available these days. I have had some fogging of spray paints when spraying in high humidity but I can't really control the mix in a spray can. I suspect your correct that the paint was starting to flash off too soon on your second coat, if the humidity was high the temp was high and that would accelerate the catylist in the clear.

[Len Woodruff]

Thanks guys for the info.