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How to get the best result when painting exterior wood

Sept. 8, 2022 Updated Thu., Sept. 8, 2022 at 10:38 p.m.

Finding the right paint for your wooden siding can be difficult because of how wood expands and contracts when it gets wet and dries, which is what causes paint to eventually flake off.  (Pixabay)
Finding the right paint for your wooden siding can be difficult because of how wood expands and contracts when it gets wet and dries, which is what causes paint to eventually flake off. (Pixabay)
By Tim Carter Special to the Washington Post

Not long ago in this column, I shared why it’s so hard to preserve exterior wood using any number of products. One of them was paint. You may have been one of the many readers who have written to me.

If I had to boil down the emails I received to just one, it would read: “Tim, I have to paint my exterior (fill in the blank). I don’t have a choice. Why do paints fail? What can I do to get the longest-lasting result? And what is the best exterior paint, in your opinion?”

Countless professional papers have been written about these topics. Scientists with Ph.D.s in chemistry spend their entire careers wrestling with these questions. That said, I’ll do my best to answer them in the limited space of this column.

First and foremost, it’s important for you to realize that common exterior house paint is really just glue with color added to it. Think about it. Glue sticks to things, right? That’s what you want: Your paint needs to stick to your house siding, windows, furniture, deck railing and so forth. And, if given the choice, you’d want it to stick for 20 or 30 years!

In a previous column, I wrote that paint and wood don’t play well together because wood has a propensity to expand and contract when it gets wet and then dries. Most paints can’t take this back-and-forth motion and eventually crack and lose their grip on the wood. Peeling paint is the result.

Have you noticed that you don’t often see paint peel from your car or from aluminum siding or garden tools? The reason is simple. Metal doesn’t expand and contract to the degree that wood does. Yes, paint will peel from metal, but the root cause is often poor preparation of the surface or cheap paint.

Glues work best when they’re applied to clean, dry, dust-free surfaces. The same is true with exterior paint. Let’s exorcise the pressure-wash demon here and now. Pressure-washing the object you’re about to paint does not get it perfectly clean.

You can test this with ease. Take your dirty car to a carwash where you can spray it with a pressure washer. Go ahead and use the soap setting, get the wand as close as you feel comfortable to the paint and rinse it. Then pull your car out of the bay and drive to the edge of the lot. Let your car dry in the sun. You’ll see there’s still a light film of dirt on the paint.

To get any exterior object really clean, you need to rub the surface with a sponge or brush using soapy water. You then rinse with clear water. This mechanical agitation gets the object clean just as your hands rubbing across your skin gets you clean in the shower.

Another cause of paint failure on exterior wood objects is the failure to pre-paint each piece of wood before the final object is built. Think of how you build a deck railing, fence, picnic table or other structure. You make all the cuts, assemble the pieces and then paint it. The problem with this approach is that at each place where one piece of wood touches another, you’ve got a tiny gap. Unless you live in the desert, water makes contact with the unpainted wood at these locations, and you know the rest of the story.

Most people don’t have the patience to cut all the pieces of wood and then prime and paint them. The end grain of each piece requires at least three coats of paint for ultimate protection. Once the paint has dried and cured for a week, assemble the structure.

Years ago, I knew this was really important. When I built my last house, I covered the entire exterior with thousands of linear feet of redwood siding and redwood trim boards. It took extra hours of work, but each time I cut a piece of siding or a trim board, I painted the cut edge of the end grain of the wood that would butt up against another piece. I didn’t paint all the sides that would be hidden, but this one simple step paid off in spades.

You can drive by this house in Cincinnati. I painted it with the best paint available at the time. It had a urethane resin. The resin is the glue component of the paint. I painted the house 25 years ago and drove past it a year ago. There’s not one place where the paint is peeling. The current owners do what I do and wash the outside of the house every two years. The paint looks as good as the day I put it on.

The last thing I want to share is the adhesive-chain issue. If you’re painting something that’s already got multiple coats of paint on it, realize that you have a chain. Each previous layer of paint is a link in the chain. While you may do everything right when applying the last coat of paint, a layer three levels down may fail because that painter didn’t do things right or used a cheap paint with inferior glue.

This is why it’s so very important to use the best paint, read the label instructions and, if the manufacturer says to prime bare wood, then do it. For best results, apply the finish paint within hours of the primer drying to get the best mechanical and chemical bond between the two linked layers.

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