A reader had a problem with the paint on his front doors, which he thought might be caused by exposure to strong sunlight for many hours each day. Fortunately, he sent pictures, which told me a different story. From the pictures, it seemed more likely that the damage was caused by moisture.
UV sunlight causes painted surfaces to fade, dull and eventually chalk. The paint on these doors appears to be in quite good shape (though maybe a little thick).
What appears to have happened is one of the most common exterior paint problems. Moisture finds gaps where it can get into the wood underneath the paint, and in end-grain situations works its way down the wood and causes the paint to separate.
This problem is very common in cross-grain situations where wood siding butts up against window or doorframes. It also occurs in situations like this one where rails butt up against stiles (vertical boards).
With these doors, water has also entered the stiles from the bottom and caused some separation.
In the case of siding butting against frames, you can keep the moisture out by applying caulk to the joints, and this is commonly done. But in the case of these doors, there’s almost nothing you can do to prevent the problem from occurring except constant maintenance or build an overhang to keep rain from wetting the doors.
The problem is that the boards will shrink and expand across the grain but not along the grain with humidity changes. Here, the sun exposure could have some impact by drying out the wood so it shrinks a little more, but the sun is by far the minor factor. It’s humidity changes from winter to spring and summer and back that has the most impact.
What to do now with these doors? Here’s what I suggested.
Sand or scrape level the raised areas of paint and any parts of the moldings where the paint is peeling, dig the paint and crud out of the gaps in the cross-grain joints, fill them with wood putty, sand the putty level, put a coat of primer over all the areas where you have removed paint, then repaint the entire doors.
For the bottom of the styles, to avoid removing the heavy doors, work some sandpaper underneath and sand lightly just to clean the surfaces. Then soak a cloth or paper towel with a thin finish such as wiping varnish (varnish or polyurethane thinned about half with mineral spirits) and slide it underneath, protecting the floor with plastic or a very thin sheet of metal. Let the thinned varnish wick up into the end grain, sealing it from moisture penetration.
As for the type of paint to use, oil-base or water-based, it’s best to stick with the same type that is now on the doors. To test the paint, dab on a little acetone, lacquer thinner, xylene or toluene. Each of these solvents will cause water-based paint to smudge and get sticky pretty quickly, within seconds. They won’t cause this on oil-based paint, but they could cause the paint to blister if you leave the solvent in contact for a long time.
Sanding, scraping, filling and repainting aren’t going to solve the problem permanently, because the wood still shrinks and expands. So gaps where moisture can penetrate will still open up and have to be kept filled. The alternative of stripping and repainting isn’t going to solve the problem either because new paint won’t stop the gaps from opening up.