Water damage to the sills and “brick mold casings. The first sign of water damage to your windows will often be on the sill. Typically this is caused by water becoming trapped between the window and the storm window. Seep holes at the bottom of the storm window frames should never be caulked. Accumulation of debris can also prevent proper drainage. Rot can become extensive before it is observed from outside, and multiple layers of paint may hold their shape despite the softness of the rotted window underneath.
A wire brush on a cordless drill is useful for removing If the rest of the window frame and the sashes are in good shape, it is sometimes possible to repair the sill. It is important to remove all rotted or suspect wood.
One method I use is to cut the sill back two or three inches, then glue and screw a piece of pressure-treated pine to the remaining section.
When there are just a few rotted areas, they can be filled with plastic body filler (Bondo). Use a hammer and chisel to remove the bulk of the rot. A burring tool or rotary wire brush on a cordless drill helps to dig further in and cleans the hole for the next step. Make sure that you remove all loose and rotted wood so that the Bondo will bond and also to prevent further damage.
Brush Minwax Wood Hardener onto all surfaces. It soaks into softer materials, so continue brushing it in. This liquid will harden fairly quickly, but I recommend waiting overnight so that the liquid hardener is completely dry before beginning the next step.
Plastic body filler can be purchased at Auto repair shops. It is a two-part epoxy type material designed for fixing dents in cars but is an excellent wood filler as well. The product comes with a small tube of hardener. Make sure that everything is ready. Put on a pair of disposable gloves.
Mix only the amount you can apply in one minute on a disposable flat piece of wood or cardboard, thoroughly mixing the two components (approximately 1″ of hardener from the tube for a golf ball size amount of body filler. Quickly apply with putty knives or Bondo spreaders to the gouged-out surfaces, and don’t even think about making the first coat perfect or smooth.
The mixture will begin to harden in about a minute on hot days, several minutes on cooler days. Once the hardening begins, you must stop and dispose of remaining material– it is now useless. Clean your putty knives by scraping against each other.
In a couple of minutes the body filler will be the consistency of hard cheese.
Use a “Surform” plane to scrape and begin the initial shaping. Note that the applied material gets quite warm during the hardening process. You may need to apply additional layers of Bondo, each time planing the extra material off to approximate the shape of the wood. On the edges of the sill it is helpful to attach a piece of wood to act as a dam to prevent the wet body filler from sagging or dripping. Wrap the wood block in plastic to prevent the Bondo from sticking.
After about 5 minutes the material will be hard enough to sand with an orbital sander or belt sander. I recommend attaching the sander to a vacuum for dust collection. Wear a good quality dust mask. Prime the wood and Bondo, then apply two coats of exterior latex house paint or enamel. The finished product will be indistinguishable from wood and will last much longer if you have been careful to remove all rotted wood and have fixed the initial cause of the rot.
Plastics, including bondo, expand and shrink with temperature. Wood on the other hand expands and shrinks with humidity. This will probably cause the bondo repair to eventually separate a bit from the wood surface, and will need occasional maintenance and repainting. The best solution is, of course, replacement rather than repair.