Humidity a concern?

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PostPosted: Sat Oct 01, 2022 8:55 am
Often times the runs or dry spray issues are caused by having a spray gun that requires 9-10 CFM of air, while using an Air Compressor that only puts out 5-7 CFM.

Without the proper volume of air, the gun isn't atomizing the paint/clear properly, so the droplets are too big = runs or too small = dry spray. It is difficult, if not impossible, to get a consistent spray because when the compressor is fully charged it starts out okay but then changes fairly quickly as the compressor cannot keep up.
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PostPosted: Sat Oct 01, 2022 10:51 am
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PostPosted: Sat Oct 01, 2022 3:00 pm
[quote="Ahajmano"]From my research and limited experience:

Sealer: if its epoxy, high humidity should not effect the curing process but will effect how quickly the solvent will flash between coats. Either spray a single wet-coat, or wait longer (15 min for example) between coats.

Urethane: Humidity ACCELERATES the curing or 2X urethanes. If you are OK with an orange peel finish, then no worries. Just look out for condensate as others have mentioned, and you may have more condensate push through your gun's air nozzle as well (unless you have a really good means of cooling/capturing it).

This was most noticable for me when I sprayed my single-stage 2X semi-gloss black in my interior.

When humidity is high, the acceleration of the cross-linking can occur before the solvents can evaporate, especially on the previous layer. You could experiment on using faster solvents and waiting longer between coats. Lighter coats help too.

Thanks, so if high humidity can speed up curing due to increased cross linking, what is cross linking? If evaporation of solvents is retarded, wouldn’t that prevent dry paint from hitting the body, causing orange peel?
Im starting to think I need to keep an eye on the weather report for humidity because it seems to go up after it quits raining and the sun comes out



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PostPosted: Sun Oct 09, 2022 8:16 pm
Cross-linking is just short-hand for the polymerization of the liquid urethane. Dry-spray doesn’t cause orange peel. Dry-spray is it’s own problem entirely, which causes a rough surface like sandpaper.

As other most experienced folks mentioned, you need to adjust the reducer speed and play with the hardener when spraying in high humidity (relative to baseline, say 30-50%RH). I’m not as experienced with playing the chemistry, so I just repeat my decent results by spraying on an average 70-80 degree day with anything between 30-50%RH. I have gotten used to spraying in much warmer temperatures but that took practice and I didn’t have a choice. The fluid viscosity drops (becomes thinner) when it’s warm, and the solvents tend to flash much faster. Easier to get runs if you add too much reducer, so make sure you practice with the right temp reducer.

Orange peel comes from solvents flashing through the urethane while it’s curing (becomes tacky). So this can happen if you don’t wait long enough for the solvent to flahs between layers, or the urethane is curing too quickly, or both.
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