Q: Help me, Tim. I’m so darned frustrated. The paint on the outside of my home and shed keeps peeling. Both have wood siding. Every few years I’m out there scraping and starting over. What am I doing wrong? I follow the directions on the label of the paint can to the letter. Is it the paint? Is it something I’m doing wrong? Is it just the weather where I live? I’m beginning to think an evil hex was cast upon my house by an enemy of a former owner. Please help me because I don’t want to put up vinyl siding to eliminate the paint issue. — Debbie S., Waterloo, Iowa

A: A friend of mine here in New Hampshire has a problem with paint on some cottages he owns. Peeling paint is a burr under the saddle of many, I’m afraid, and the causes are numerous. I don’t know that I can cover all of them in the small amount of space provided for this column. Let’s get started.

I feel the best way to understand why paint peels is to step back and think about all the things you know of that are painted and seem to hold the paint without peeling. How about your car or truck? What about street signs? What about painted aluminum siding, gutters and downspouts? How about lawn mowers, snowblowers and many garden tools you might own?

Do you know someone who has a house covered with fiber cement siding? My house has it. There’s not one place where my paint is peeling. My neighbor’s house has the same siding. Both houses are 20 years old, and there’s not a speck of peeling paint in our harsh New Hampshire weather.

The one thing in common with everything I’ve just mentioned is that the paint is applied to things that are not wood. Wood is a very interesting material, but it doesn’t play well with most paints unless you paint it correctly before it’s installed.

Wood is hygroscopic. This means that water can easily enter wood either as a liquid or a vapor. The issue is when most wood gets wet, it swells and expands in size. Metal and fiber cement siding don’t change shape when they get wet. This is very important.

Now let’s think about paint. What is paint in its most basic form? Paint, for the most part, is just colored glue. The chemistry of most paints is not too far from common white and yellow glue or caulk that you might purchase at a paint store or home center. My original business partner and I used to paint homes in the summer in college. He went on to get a PhD in chemistry where he worked for over 30 years for the largest paint company in the United States. Over the years, he has taught me much about paint.

Yes, I realize I’m oversimplifying it, but think about paint. It’s made to stick to things just as glue adheres to things. Paint can have all sorts of other characteristics to minimize color fade, gloss and so forth, but when the dust settles, its main job is to stay stuck to the things it covers.

Here’s the rub. If the wood that’s under paint moves too much — that is, expands and contracts as it gets wet and dries — this can be more than the paint can handle. When the bond breaks between the wood and the paint film, the paint peels.

Paint can also be pushed off wood by the vapor pressure under the paint. If wood gets wet, and the sun heats up the wood siding, the water in the wood can turn to vapor, creating a blister. This vapor pressure can easily pop the paint right off the siding. You can demonstrate this by applying a thin coating of flour and water to a balloon. Let it dry and blow up the balloon, and the flour pops off.

Debbie did all she was supposed to by following the paint label instructions. Do you? Do you actually wash the surface you are going to paint like you wash your car or dishes? You need to rub the surface with soapy water, rinse and allow to dry. Do you scrape off all other loose paint? You wouldn’t put on fresh clothes to go out to dinner after sweating all day working in dirt and grime, right? You’d take a shower. So make sure your surface is squeaky clean.

Painting at the right time of day can play a part. It’s best to follow the sun. This means to paint in the shade, not in direct sunlight. It’s best if the paint dries slowly. Hot breezy days are perhaps the worst time to paint, in my opinion.

If you want the paint to stick well to wood for many years, you need to shrink wrap it in the paint. This means you need to paint the wood on all sides and edges. You can pre-paint wood siding and trim with primer and finish paint and then install it.

However, if you cut siding or trim to length, you then need to paint the cut end. Most carpenters are reluctant to do this because it really slows them down and it can be a mess. But the cut end of siding and trim is the place where water enters wood the easiest. These cut ends must be sealed if you want to prevent peeling.

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