Maybe you’re painting a deck. Or in my case, a goat milking stand. But since these things live outdoors, its worth asking, will paint waterproof the wood?
When well-maintained, paint seals wood and protects it from water, effectively waterproofing it. However, when cracks appear in the paint, water can reach the wood, and the wood is no longer waterproof.
However, there are a few things you can do to increase the longevity of your paint job, and the ability of the paint to protect the wood.
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Why Paint Waterproofs Wood (and Eventually Fails)
It’s probably pretty obvious – paint creates a barrier between wood and the world. When water hits the wood, the layers of paint prevent the water from reaching the wood.
Paint isn’t affected by water like wood is, so no harm done! Yay.
But here’s the thing: Wood moves.
And it moves more than you realize. At least, more than I realized until I started making wood projects, and my perfectly-aligned pieces were no longer perfectly aligned when summer came around.
Here’s what the edge of my coffee table looks like now:
As you can see, the center pieces stick out a clear 1/8″. I didn’t build it this way, promise.
My shop countertops are another great example. I filled the gaps between the pieces of wood with caulk and resin in the winter. It’s summer now, and look how the caulk has squeezed out the material:
So wood moves. Great. But what does this have to do with paint?
Well, paint is rigid. When wood moves, the paint doesn’t necessarily move with it. It can handle the movement for a while, but as the paint ages it becomes less flexible, and eventually when the wood moves, the paint cracks.
And once the paint cracks, water can get to the wood. Bad news.
So basically, there are two ways to help paint waterproof wood for longer: reduce wood movement, and increase paint durability. Lets talk more about that.
Tips to Increase the Longevity of Your Waterproofing Paint Job
So before I start, I want to give a quick FYI – sometimes, the easiest thing to do is just repaint the wood when the paint starts to crack.
I’m going to give a list of things you can do to reduce cracking and make the initial paint job last longer, but in many cases, just putting on another coat when the paint cracks is the best option.
Tip 1: Paint the Entire Piece
This sounds obvious. Like, you plan to paint the whole thing anyway, right? But what I really mean is to paint the wood that might not be exposed.
If you were only going to paint the top of the table, paint the bottom of it too.
Wood expands because it absorbs moisture from the air. That’s why wood grows in the summer; there tends to be more moisture in the air.
If you can block the air from the wood on all sides, not just the side that’s exposed to the human eye, you’ll reduce the amount of moisture that can be absorbed, and therefore reduce wood movement.
This will also help reduce warp in your wood, because warp is caused by uneven moisture absorption.
Tip 2: Prime Before Painting
Primer has two purposes: to seal the wood before painting, and to help the paint adhere to the wood.
Both of these aspects will help your paint waterproof the wood for longer.
Sealing the wood helps reduce wood movement, and increasing adhesion reduces chips and cracks that allow water to access the wood.
The primer you use depends on your project, but I’d avoid water-based primer if you’re worried about waterproofing wood. You want something a little more heavy-duty than that.
I have a whole post about choosing a primer here, but generally I’d recommend either Zinsser’s Oil-Based primer, or if you’re really worried about sealing the wood, Zinsser’s Shellac-Based Primer.
Tip 3: Use a High-Quality Exterior Paint
Exterior paint is formulated to remain durable in tough weather conditions in way that interior paint is not. If you’re worried about waterproofing, that’s the paint you want.
The higher the quality, the more durable the paint will be. I’m not talking about brand (personally, I (and Consumer Reports) have found that Behr is just as good as Sherwin Williams or Benjamin Moore,) but more the different options within a paint line.
If you’re worried about waterproofing and long-term durability, you want the best option they offer.
It’s also worth noting that you don’t want a topcoat. Exterior paint doesn’t require a topcoat, and frankly, anything that you’d use is probably lesser quality than the paint itself.
Oil-based polyurethane is the strongest topcoat, but it shouldn’t be put over paint because it yellows. Water-based polyurethane technically can go over paint (and won’t yellow,) but the exterior paint does such a good job on its own that it’s not really worth your time.