I second the others--for a canopy that's otherwise clean, a dip in Future helps. The clear acrylic liquid fills the tiny scratches and pits on the surface.
For a canopy that's damaged--glue smudge, say, or scratch--I also use polishing pads, I think mine are by Tamiya. They go from 1800 grit, I think, up to 3200? I'd have to check to be sure. That's followed by a dip in Future. The canopy comes out good as new.