Before You Begin
The joint between the window jamb and windowsill is a breeding spot for wood rot. Before you paint over this troublesome area, make a lasting repair with a two-part epoxy wood system that often comes sold in a repair kit. This is a small project and the repair kits have enough product to make several applications, so you can use them and the skill you develop to repair other damaged wood around the house.
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Step 1 : Expose the rotten area
Use a hook-type paint scraper to remove as much of the old paint from the window jamb and sill as possible to expose the wood. You may also find wood rot or damaged wood in others areas of the window. The bad areas will usually be stained dark and feel soft to the touch.
Step 2 : Chisel out all rotten wood
If you can push the blade of a screwdriver into the wood without really trying, the wood is bad and should be removed. Use a chisel to gouge out any soft wood. In our case the bottom of the window jamb and parts of the sill were damaged. Remove as much bad wood as possible.
Step 3 : Mix and apply the epoxy
If there is extensive rot you will get a longer lasting repair by drilling a series of 3/8-inch holes into the bad area after you have chiseled out the bad wood. Then apply a wood consolidant and let it dry. This product will be included in the repair kit or can be purchased where you find the epoxy.
Mix the epoxy filler according to the manufacturer's directions and use a putty knife to fill the voids. Overfill the voids so you can file and sand the repair area to the original shape of the sill or jamb.
Step 4 : Shape and sand the repair
Allow the epoxy filler to harden for at least 4 hours, longer if it the temperature is cool outside. In some cases you may have to wait overnight. Then use a course file or rasp to shape the repaired surface to the existing profile of the window sill or jamb. Flat areas should be sanded smooth with sandpaper wrapped around a scrap of wood or with a sanding block.
Step 5 : Topcoat with paint
Apply a good quality exterior primer to the patched area and the entire window. Then follow up with a couple of coats of good quality exterior paint.