Remove the windows from their installation areas. Tilt the windows as though you were going to clean the exteriors, and then release the metallic springs that hold them in place to release the sashes and glass.
Place the windows in a well-ventilated work area, such as outdoors or in an open garage. Never use paint and other chemical products in a poorly ventilated area, as fumes can cause illness.
Unscrew the window locking hardware with a screwdriver. Take note of the location and direction of each piece of hardware so that you can easily reinstall the pieces after painting. Place the screws and hardware in a safe place while you work.
Sand all wooden sash areas with 120-grit sandpaper. Sanding will remove surface dust and dirt, and will also prepare the wood for the new stain or paint. Take care not to touch the glass, as the sandpaper can easily scratch the glass. If you need to, mask off glass areas with masking tape and protective paper or plastic.
Clean off sanding dust by spraying the wood and glass with compressed air from a machine or can. You can also wipe down all sanded areas with a tack cloth to collect excess dust.
Apply the first coat of stain, or the wood primer if you are using paint. For all stain and paint products, you can use a paint gun or a paintbrush to apply the product. Allow the first coat to dry and apply a second coat if necessary, per product instructions. Allow all initial stain or primer to dry completely before continuing. Consult specific product instructions regarding drying time.
Dampen a paper towel with paint thinner, acetone or another solvent and wipe away any overspray on the glass windows. You can do this throughout the painting project if you accidentally paint on the glass.
Apply the first coat of urethane or finishing paint with a spray gun or paintbrush. Allow this first coat to dry completely before continuing.
Sand the newly stained or painted area with 220-grit sandpaper to ensure a good surface for the final coat. Blow away sanding dust with compressed air or wipe it away with a tack cloth.
Apply the second coat of urethane or paint to the windows. Allow this coat to dry completely and inspect your work. Touch up areas or apply a third coat if necessary to achieve your desired color or consistency.
Clean the glass with solvent or scrape it with the flat edge of a razor blade to remove any excess paint or stain. Reinstall all locking hardware and put the windows back into place in your home.