The only thing wrong with them is that they leak all the way around -- around the edges, where the windows meet each other when closed or open. They are 50 year old aluminum frames, single pane, with no vinyl finish, but otherwise look like this... http://www.homedepot.com/p/JELD-WEN-V-4500-Series-Left-Hand-Slider-Vinyl- Windows-60-in-x-48-in-White-with-LowE-Glass-8A2274/203645522
The seals between the glass and the frame are OK. It's the aluminum-to-aluminum contact points that leak.
And while I'm asking... if I can seal them properly, the next thing would be any tips on painting them white or some other type finish so they match the new ones as closely as practical?