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Is it possible to paint a house with rain in the forecast? Our forum discusses their experiences with painting in humid conditions, and even touches on the use of primer as well.
Original Post: Drying time needed for exterior latex paint before rain
I'm branching out to exterior painting, and I'm concerned about painting when rain is expected within 24 hours. I operate in the Philadelphia, PA suburbs. We get a fair amount of rain in the spring. In the summer, we can go for weeks with afternoon and evening thundershowers expected every day. Most latex paints I use dry in about 4 hours. But, what happens if rain blows in after 5 or 6 or 8 hours?
Conventional wisdom says if you've got 4-6 of good dry time, you should be OK. The paint and humidity will affect this, as will the amount of rain and splash-back, and the vertical vs. horizontal surfaces, etc. If it's not quite dry enough, or it rains too hard too soon, the surface may look pretty bad and you may have to sand and repaint.
Slickshift hit on a key word: humidity. That is the most important thing. I painted the trim on a cedar house up on pilings on the beach 25 years ago, and did so just prior to the fog rolling in. I think there already was some fog in the air when I was painting and it just got worse and worse. The next day I returned to the house and all the trim I had painted the evening before was nowhere to be found. It erased. It didn't even run down on the silvery cedar siding and wreck it (luckily). It so thoroughly washed off it was as if I dreamed I had painted the trim.
There are many, many variables. Humidity is a big factor, but don't forget what type of substrate, paint, method, and thickness are applied. And then you need to factor in sun and wind (or lack of). Each job will be different. I have had jobs get rained on 1 hour after completion with no ill effects and have had jobs that dried for 8 hours that required a lot of fixing. All you can do is use your best judgement, hope for the best, and play the cards you are dealt.
Now what about the drying time of exterior primer before rain? Would that be the same, 4 to 6 hours, or does primer have different requirements to be safe enough to be exposed to rain and other humidity? In my case my project is small and can be moved, but the end result will be having it outside year-round.
Primer's not as durable as paint, but I would think a light rain several hours later would pose little threat. I paint outside, but the garage is always empty if I think there's any chance of rain so I can get the project out of the rain while it's drying.
There are many factors that determine the drying time of the primer. Different primers dry at different rates—read the label! Next, weather conditions play a part. Primer/paint will dry quicker in hot dry weather; cool temps or high humidity can slow down the drying process. Wind can also speed up drying some. How dry/sealed the substrate is also plays a part—dry or porous substrate will suck up the primer quicker resulting in quicker drying time.
An hour or so after the primer is dry to the touch should be enough time for latex primer to fend off light rain. A hard rain will affect the primer if it's not good and dry. If the primer appears to be compromised, sand and re-prime.
Oil based primer takes longer to dry, but can withstand rain fairly soon. As long as the substrate is dry when the primer is applied, rain (but not a gully washer) won't affect it too much 30 minutes after it's applied. Rain will slow down the drying time and might affect the look, but shouldn't hurt its bond with the substrate.
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