When painting a wooden window, a painter should open it slightly and, just after the paint is dry enough to touch, move the window up and down to break the paint seal between the sash and the frame. Many painters overlook this procedure, and many wooden windows get sealed shut as a result. To open a sealed window, it helps to have a triangular sawlike tool called a deglazing tool for breaking seals and cutting nails, but you can usually do the job with more conventional tools.
Break the Paint Seal and Pry
Run a deglazing tool, metal putty knife or utility knife around the perimeter of the window to break the seal between the window sash and the jamb. If you're using a putty knife, tapping with a hammer helps the blade break through the paint. Tap lightly -- you don't want to jar the window and break the glass.
Try wiggling the window back and forth with your hands. It may help to work a putty knife between the sash and jamb at several points and pry.
Go outside and repeat the procedure on the outside of the window, if you can't get the window to move from the inside.
Tap a pry bar between the bottom of the window and the sill and pry. If the window won't move, change the position of the pry bar and try again. Once the window moves, pry slowly and carefully -- changing the position of the pry bar frequently -- to avoid putting stress on the glass. Once the window opens about an inch, go inside and lift it open by hand.
Remove the Window Stop
If the window is on an upper floor, it may not be safe to work from outside without erecting a scaffold or securing a tall ladder to the siding. In that case, you can use an alternate procedure to break the paint seal, but it will involve some paint touch-up.
Tap a putty knife into between the window stop and the jamb. The window stop is the piece of trim that prevents the window from falling forward. Pry the bottom section of the stop out with the putty knife, then move the knife gradually to the top, prying as you go.
Work a deglazing tool behind the stop and cut any nails holding it to the frame. It may also help to undercut the stop using this tool. When the stop is free, remove it carefully to prevent damaging the miter joint at the top.
Wiggle the window to loosen the paint holding it. It may break free, and if it does, lift it up and down a few times to make sure it slides well. If you can't break it free, remove the stop from the other jamb.