What should I do about polyurethane coating on wood mirror frame so I can paint it?
Lightly sand the surface with 150 grit sandpaper or a medium/fine sanding sponge to roughen it. Now wipe it down with warm water and a mid soap to remove any dust and have a nice clean surface. All that's left is priming. Use an oil base primer, you could use Kilz or Zinsser Cover Stain. After the primer has dried you can go ahead and paint.