Q: My house was built in 1954. It has wooden windowsills in the bath that get wet when someone showers. Areas of rotted wood have been repaired, I think with DAP Plastic Wood. The windowsills have been painted with at least two coats of oil-based paint. Recently, I had new windows installed, which broke the seal around the window. The paint is now peeling off, and I need to repaint. The window installer said that happened because the sills weren't painted with primer first. I always wipe the sills dry after showering, and I open the window to let the humidity out. For now, I'm also taping plastic wrap over the sills when I shower. What are the best practices for fixing the damaged wood and painting the windowsills to protect the wood from shower water?

Silver Spring, Md.

A: A window in a shower seemed like a good idea in the decades before efficient bathroom fans became common, but gaps in the window trim, especially between the window and the sill, can let water get into the wall and rot out the framing. The moisture can even travel down the wall and into the floor supports, rotting that out, too.

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Before you focus on how to patch the wood and repaint, call a house inspector and ask to have the shower walls checked for moisture intrusion. Make it clear that you aren’t selling and don’t need a fancy written report; you just want advice about this particular situation. That might get you a better deal than a full inspection, which at many companies costs about $350. Moisture meters start around $50 at home centers (for example, Home Depot sells a General Tools model for $54.97). Checking a tiled shower wall is tricky, and a professional’s experience with building problems adds a lot to an inspection.

You can, however, do a few preliminary checks. Are all of the tiles flat against the wall, or do some bulge out? Are any loose, especially under the window? Test any suspect tile by trying to pull it off with your fingers. It should be impossible. If one or more tiles come loose, that’s almost a sure sign of a moisture problem. It might also indicate that the tiles are attached to a kind of drywall known as greenboard (for the color of its paper facing). Greenboard is moisture-resistant but not moisture-proof. In many houses built in the 1950s and later, it was used behind tile in showers, which no smart builder would do today. Because the grout between tiles isn’t waterproof, moisture can eventually get through, degrade the drywall and cause the tiles to come loose. Having a leak from a window that isn’t completely sealed just adds to the risk.

If you can pull off tiles, or if an inspection shows water intrusion, you may need to redo the tile so the moisture doesn’t rot out the framing of your house — probably not something you want to know. But redoing the tile would give you a way to effectively seal the window and windowsill. Fine Homebuilding magazine’s website has a great illustration and tips, which you can find by typing “window in tiled shower” into the search box at FineHomebuilding.com. One tip: Install a windowsill made of Corian, a countertop material that’s impervious to moisture and doesn’t need painting. Unfortunately, there is no way to install the recommended flashing without redoing at least some of the tile, because the flashing has to wrap down behind the tile. An alternative that wouldn’t involve redoing the tile would be to wrap the flashing over the top edge of the tile along the windowsill and then cover that by attaching a small piece of molding to the bottom of the windowsill where it overhangs the tile.

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If you decide just to patch the wood and repaint, first scrape off the loose paint and dig out any soft, rotted wood with a sharp tool. Brush on wood hardener to solidify spongy wood fibers, creating a firm base. When that dries, fill holes with a wood filler that doesn’t shrink as it dries. One combination of products is Bondo Wood Restorer as the hardener (eight ounces for $18.97 at Home Depot) and Bondo Wood Filler, a two-part polyester formula (12 ounces for $13.68).

Oil paint is more impervious to water than latex paint, so it might seem to be the best option for a windowsill in a shower. However, testing done by the U.S. Forest Service’s research center in Madison, Wis., on wooden decks (a good stand-in for a windowsill in a shower) showed that when water gets through, as it inevitably does, oil paint keeps the moisture from evaporating, so the wood is actually more prone to rotting. Water-based paint sheds liquid water but lets water vapor through, allowing the wood to dry faster.

Prime before you paint. To ensure that the primer bonds to the wood and to any existing paint, lightly scuff up the surface first. Because your house dates to 1954, avoid sanding unless you are certain the existing paint is lead-free. Instead, wipe the surfaces with a deglosser, such as Klean Strip Liquid Sandpaper Cleaner & Deglosser ($8.97 for a quart at Home Depot). It’s okay to use water-based products on top of oil paint.

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In the interim, rather than trying to cover the windowsill with plastic wrap before each shower, rig up a curtain rod and a plastic curtain that moves easily on rings. Make the curtain long enough to cover the windowsill. Close the curtain when people shower; push it to the side afterward so you can open the window. Even after repairs, you might want to keep using the curtain: It’s cheap insurance against future problems.

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