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Sun, moisture, and weather are hard on windows. Water might condense on the glass in winter and end up on the sill. Or you may forget to close the window when it rains. Sun streams in and causes the water-weakened finish to buckle, and the finish degrades over time. Eventually, it is necessary to sand and strip either the whole window, or parts of it.
Before the 1950s, the clear finish on most windows was shellac. Lacquers came after that, and now polyurethane coatings are almost standard. Fortunately, you don't need to know exactly what's on your windows because polyurethane will almost always do a good job of covering anysurface. You have a choice of oil or waterbase poly; both work satisfactorily. Exterior spar varnish (made for boats) is especially durable and it can also be used inside.
Can you strip a painted window and then stain and varnish it? Yes, but it's hard to do well. You'll need to coax paint from the crevices of the trim, the window frame, and all the molding. It might be easier to remove the woodwork and send it out to a dip-and-strip shop than to do it yourself.
Paint and varnish remover, stain (optional), clear finish, 120- and 180-grit sandpaper, wood putty, painter"s masking tape
Evaluate the present finish.
If the window is in fairly good condition, you may only need to sand it enough to smooth it and dull the sheen. Begin with 120-grit sandpaper; then switch to 180-grit sandpaper for a smoother surface. If the finish is badly damaged or built up, it's best to remove it. On many newer windows the sashes pop out, but you may have to carefully pry off trim pieces to remove sashes on older windows. Cover the work surface with an inexpensive drop cloth. Brush paint and varnish remover onto the wood following the manufacturer's instructions. (Provide good ventilation, and wear rubber gloves and a vapor respirator.) Scrape off the finish with a putty knife. Rinse with the solvent recommended for the remover.
Fill holes and gouges.
Buy a commercial wood putty that closely matches the stain you're using, or choose a stainable putty. Fill the flaws with wood putty, spot-sanding it when it dries. Remove the dust with a tack cloth (if you'll be using an oil-base finish) or damp paper towel (if using a water-base finish). If you stain the wood, either tinted or stainable putty comes close to a match, but it may not be perfect. If you're not going to stain, make your own putty: Collect a few pinches of fine sawdust from the type of wood you'll be patching, mix with five minute epoxy, and fill in any holes. When the epoxy dries, trim off the excess with a utility knife or sharp chisel and then sand. For an extra-smooth finish do the final sanding with steel wool.
Mask the panes, and also the sides of the sash and the tracks.
Masking with tape takes time, but it saves work later. Low-tack painter's tape is easy to remove and leaves little residue; it works well for masking off trim too. There are several types of low-tack tape so read the label carefully and choose the one that best meets your needs. Make sure you mask the two side edges of the sash and also the track; these parts are almost always left unfinished so the window opens and closes smoothly.
Apply the stain and finish.
Staining the wood will darken or warm its color, but the color will look even only if you've been thorough in removing the old finish. Windows made of pine or another softwood often absorb stain unevenly, causing a blotchy effect. Avoid this by using a gel stain, which won't blotch because it doesn't soak deeply into the wood. Let the stain dry, and apply at least two finish coats (three coats on the sill, which takes the greatest beating). Sand lightly between coats, removing the dust with a rag dipped in denatured alcohol.
Remove the masking tape.
Remove the tape from the glass as soon as the finish begins to dry. If you haven't taped off the glass (or if your masking job wasn't quite adequate), draw a sharp utility knife along the edges of each pane, leaving a narrow margin of finish on the glass. Push a window scraper or a single-edge razor blade toward this cut line to remove finish from the glass.
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