Long term, what happens if I paint wet wood?

Question

If I paint wood, for outdoor use, while it is still very wet, what problems can I expect in the long run (2-3 years)?

In my specific case, I have pressure treated outdoor lumber (fir), which I purchased at Home Depot (where my only theory is that they store their wood in tanks of water before putting it on the shelves). The wood also feels relatively fresh based on the way it feels when I rip pieces off along the grain. The paint is an exterior latex paint with primer (Behr Exterior Premium Plus Ultra). I intend to paint every side, which will completely seal it with paint.

But it's going to take weeks or months for this wood to dry out and I only really have a day or two.

I don't mind repainting in a year if it is just a matter of the paint not sticking. My main concern is effects on the wood itself due to trapped moisture and pressure treatment chemicals.

Answer 1

A paint chemist might have a cow over my answer, but I will give it a try.

Regardless of what finish you put on the wood, the surface must be dry. In order to bond with the wood, the finish must be absorbed. If the surface is too wet an oil based finish will be rejected (oil and water don't mix). If the finish is water based the excess water in the wood grain will prevent the finish from sinking onto the surface. In either case the finish will cure without getting a good grip on the wood fibers and the finish will tend to peel off in short order. A simple test to see if the surface is dry enough is to sprinkle some water on the surface of the wood. If it beads up, the wood is too wet. If it is readily absorbed then you can proceed with finishing.

The next concern is how much water is inside the wood. Treated lumber is made using water infused with chemicals to prevent mold growth and insects. That is why it is so wet. The wood goes directly from treatment to the lumber yard... no need to wait any longer. This means that the wood will be evaporating water for some time (6 mos. to 1 year at a guess). If the wood is not finished on all sides, the water has a path to get out, although warp and twist from unequal drying are possible. If all surfaces are well sealed then the water vapor will be more likely to blister the surface, finding the thinnest/weakest locations to break through as pressure increases inside.

I do not think the treatment chemicals in the wood will have an effect since they are present whether the wood is wet or dry. There may be a possibility that they can be absorbed by the paint and affect its performance or color. However that is just conjecture on my part. A test sample should alleviate any concerns here.

Answer 2

The usual advice from the lumber yard is to wait a year before painting pressure treated wood. It's really CMA advice which assures that the wood will be ready to paint and avoid the pitfalls of painting wet wood - it should be dry within in a year, no matter what happens.

However, this source (many pop-up when you Google "painting pressure treated wood") gives a concise 5 step process for painting pressure treated wood.

• Wash the surface with soap and a stiff brush and rinse.
• Wait for it to dry (possibly weeks or months) - check for dryness by sprinkling with a water - if it beads-up, it's still wet. If it soaks in, you're good to go.
• Prime the surface with primer designed for exterior use on pressure treated wood.
• Let the primer dry - may take a couple of days.
• Paint

If you paint the wood before it is dry you will surely regret it. The paint will blister and peel, but will not peel completely. You will be faced with scraping and extensive unpleasant surface preparation before you repaint. Some things simply should not be rushed.

Answer 3

If I paint wood, for outdoor use, while it is still very wet, what problems can I expect in the long run (2-3 years)?

I think the issues that should concern you are more short term than long term, and that is failure of the paint and not a problem with the wood.

However, I think this might be a case where nobody can tell you for sure what's going to happen. Although obviously being cautious and waiting wouldn't be a bad idea, there's nothing to say for sure painting early the paint will fail. It's just more likely, not a sure thing.

Advice on this front, whether based on individual experience, book learning or manufacturer literature, will largely agree that the shouldn't paint onto a wet surface. But what does 'wet' actually mean? Wet and dry are in many ways relative terms, not absolutes. For example say you were working with wood that's at equilibrium with the local environment, of course that environment dictates how dry the wood actually is. So quite obviously in Georgia wood is significantly higher in moisture than in Arizona. The EMC in the first case could be well over double that in the second, yet in both places paint can perform as it's intended to.

I intend to paint every side, which will completely seal it with paint.

That's 'seal', not seal.

Only a coating that actually provides a good moisture barrier should be thought to seal a surface (and that is with a uniform, relatively thick, application).

"Latex" paint (terrible name — doesn't contain latex and never has!) is flexible and highly moisture-permeable, so actually it's an ideal paint to use if you want to paint wood that's not as dry as you'd wish it to be.

That's in respect to the wood and allowing it to continue to dry, but all waterbased paints rely on going on to relatively dry surfaces to bond well. This doesn't mean the wood has to be dry through-and-through, but the surface at least should be fairly dry or it will retard the drying of the paint and could significantly compromise bond strength and/or film strength (somewhat similar to painting at temperatures below the low end of the recommended range).

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Slightly off-topic so I'll tack this on last:

The paint is an exterior latex paint with primer (Behr Exterior Premium Plus Ultra).

I would be wary of a product that is said to combine the qualities of primer and paint in one. All of them are compromises of one sort or another and I can assure you that in nearly every case that they cannot match the performance of a true primer followed by paint.