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Why Exterior Paint Peels

Untitled - October 19, 2022
Untitled - October 19, 2022
peeling exterior paint on wood siding

Why Exterior Paint Peels | The reasons why this exterior paint is peeling are as plentiful as bread toast crumbs on a kitchen counter. Copyright 2022 Tim Carter

Why Exterior Paint Peels

Two weeks 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 that reached out to me.

If I had to boil down all the emails I received to just one, it would have said, “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?”

Hoo Boy! Do you realize countless professional papers have been written about these topics? Did you know that people with PhDs in chemistry spend their entire careers wrestling with these questions? I know as one of them was a close friend of mine back in college. That said, I’m going to do my best to share the most that I can in the limited space of this column. Let’s get started.

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 - you want your paint to stick to your house siding, your windows, your furniture, deck railing, etc. And if given the choice, you’d want it to do so for say 20 or 30 years!

Two weeks ago I shared with you 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, aluminum siding, wheelbarrows, etc.? The reason is simple. Metal doesn’t expand and contract to the degree wood does. Yes, paint will peel from metal, but the root cause is often based in poor preparation or cheap paint.

Glues work best when they’re applied to clean, dust-free, and dry 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 filthy car to a DIY car wash where you 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, 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. This is an indisputable fact.

One of the root causes for paint failure on exterior wood objects is the lack of pre-painting each piece of wood before the thing is built. Think of how you build a deck railing, fence, picnic table, etc. You make all the cuts, assemble the pieces, and then paint it. The issue is each place where one piece of wood touches another you’ve got a tiny crack. Unless you live in the Atacama Desert, water enters 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, then assemble them. I can see you shaking your head now!

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, Ohio. 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 he 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!

Column 1469


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