The first step in restoring older wooden window sashes and frames is to remove loose peeling paint usually with the aide of a paint scraper. Choose a scraper of appropriate size and shape for your project and keep it sharp. It's convenient to have several different scrapers to make the job easier. If you do lots of scraping you might want to consider a carbide blade which keeps its edge longer.
For more stubborn areas a variety of chemical strippers are available that will quickly cut through multiple layers of paint. Many are safe and easy for nearly anyone to use and have the added advantage of being environmentally friendly. Whichever type you choose be sure to closely follow the manufacturers instructions and give the stripper plenty of time to work its magic.
For detail work you'll find a few additional tools very helpful. These include brushes in different sizes, shapes and bristle construction. Choose nylon bristles for delicate tasks and brass or stainless steel for tougher jobs. A set of small tools like dental picks can be indispensable when cleaning out nooks and crannies.
When bare wood is exposed on your project you can determine its overall condition. Large holes, dents and imperfections should be filled prior to finishing. On painted projects select a durable, fast setting epoxy compound. If your project includes stained or clear finished wood you'll want to select the appropriate color wood filler or Shellac stick to finish over.
Ask any professional and they will quickly relate that the true secret to both the appearance and durability of any finish job is the surface preparation. Start with sandpaper coarse enough to begin smoothing any patches or to remove remaining stubborn paint or finish. Continue through progressively finer grits until a smooth clean surface is obtained.