I looked into this when i did mine,
Your options are...
Paint Removal - Nitro mors, bead/sand blast, sand down alot! Haha
Filler - Depending on how deep the scratches are, lighht ones i would use body filler, and for deeper stuff something like a chemical metal then sand that flat back.
Painting - 2 Coats of Etch Primer, 2 Coats Filler Primer, 2 - 3 Colour of your choice, Then 2 to 3 of Lacquer.
I'm sure some people have different methods, thats just how i would do it.
As I have access to compressor, sand blasting etc, i would go about that route, and maybe not even lacquer it dependant on what typer of top coat i used. I.E Acrylic, Poly, 2 Pack etc.