Deep scratches and micro cracks, delamination, are forever. Haze can be removed by polishing with a plastic polish. 3M, Mothers, etc offer plastic polishes for car parts. If you want to get really aggressive, you can attack the lens with fine grades of wet/dry sandpaper to remove the worst defects, followed by a polish. Another alternative is to clean em up, polish them down, and then coat em in clear or tinted paint for a bit of UV protection and to hide damage.

 

 

 

It depends on what you consider deep scratches. Most scratches can be sanded out. Polishing takes only a few minutes on a buff jack. As stated, if the scratches are deep enough they can't be sanded out, a quality clear will fill them, and then a light sanding to smooth out the orange peel, and buff to a high gloss.