Windows stand up to a lot of weather beating. Unless you have frames that don't need painting, like PVC frames, you will want to find the right way to paint them and to look for and deal with weather damage. To do this well takes some patience.
Exterior Painting
The most time-consuming part of maintaining single hung windows is painting the outsides of the frames. You will have to do this more often than you paint your siding but doing it well will reward you by lengthening the amount of time before the task needs to be redone.
Washing
In painting single hung windows, preparation is key. First, you need to clean the window and window frame thoroughly. You don't want to be applying the paint on top of dirt and grease. A scrubbing brush and mild detergent is ideal. A pressure washer can be too fierce and may cause leaks.
Getting rid of mold
Bleach and water in a 1:4 ratio is effective at getting rid of mold. It will at first turn white, but you can get rid of it if you leave the solution on the single hung windows a few minutes then wash it off.
Scraping and Sanding
You need to scrape off any loose paint from the frames of single hung windows. Use a rounded knife like a putty knife to get it all without damaging the frame. Be careful to really get into the edge around the glass. Also take out any loose glass. And any caulking that is loose, then sand the whole frame of the single hung windows. You can use a powder sander but you will probably need to sand by hand to get into tight areas around the glass.100-grit sandpaper is good to give you a really smooth surface to paint the frames of your single hung windows.
Of course, you will also need to do any repairs of the frames or the glazing on the single hung windows before you paint them, but this is how to prepare the wooden frame itself for painting.