Your window check should begin with a visual inspection. Look for any loose, flaking, or deteriorated paint on the sash, frame, and sill. Paint is the first line of defense against water infiltration into the wood comprising the historic window sash and frame, so it is critical to maintain it in good condition. Pay careful attention to the bottom rail and sill of the window, where moisture collects, as this is often the first area to deteriorate.
Any broken window glass will be obvious, but the condition of the glazing putty that holds the window glass in place needs to be carefully checked. If it is cracked, loose, or missing sections, it needs to be replaced. Caulk between the window frame and wall surface should also be inspected for soundness and a tight seal.
Moving beyond a visual inspection, you should check the soundness of the wood by pushing a small knife into it. If the knife enters with little resistance, a painted surface may be hiding rotten wood below. Wooden components of the window that are broken or rotted – including the muntins, rails, stiles, or portions of the frame – can all be replaced. If you are handy and have the appropriate tools, making and installing replacement parts yourself may not be terribly complicated. However, you may find it simpler to hire a window restoration specialist to do this work.
You will likely already know if your window is not operating correctly, i.e., if it has been painted shut, has broken sash cords that prevent it from staying open without a brace, or if it sticks and is difficult to open. All of these faults are repairable, although they usually require taking the sash out of the frame from the inside of the house. More information on these higher level repairs can be found in the National Park Service Preservation Brief 9 on the repair of wooden windows.
If you find defects in the paint coating, you should remove any loose or flaky paint with sandpaper or a gentle hand scraping. If the paint removal leaves you with patches of bare wood, the edges of any remaining paint should be feathered to bring them even with the bare wood.
Replacement of glazing putty must be done prior to painting. Putty removal will often require a heat source, ideally a heat gun, if the putty is very old and dry. Once warmed and softened, it can be carefully cut and chipped out using a utility knife and chisel. You must be careful not to burn, cut, or gouge the wood as you do this work. To install new putty, first soften it by rolling it into a thick rope in your hands. Press it into the joint between the frame/muntin and the glass, leaving plenty of excess. Once all four sides of the pane are filled, run a putty knife along each side of the pane at an angle. This will press the putty into the joint and cut off the excess. The putty should dry for at least 24 hours prior to painting.
Once the surface of the window is sanded smooth and the putty is dry, any bare wood should be primed. After the primer is dry, recoat the component or entire assembly, as needed, with a high quality paint. Be careful not to paint the joints between the sash and frame, as this can cause the window to bind when operated or, worse, to stick shut. When you paint the putty around the window panes, you should overpaint onto the glass slightly to create a seal.
With simple preventative maintenance, you can keep your historic wooden windows operational and in good condition for another 150 years. If your exterior window inspection reveals work that you feel is beyond your capabilities, your State Historic Preservation Office or local historical commission may maintain a list of window restoration specialists in your area.
For additional information see:
Myers, John H. Preservation Brief 9: The Repair of Historic Wooden Windows. Washington, D.C.: National Park Service, 1981.
Young, Robert A. Historic Preservation Technology. Hoboken, NJ: John Wiley & Sons, Inc., 2008.