yeah the shrink-back/bite-up is due to the million layers of paint underneath. good idea, it might actually be easier to get a pair of vented/wide S13 fenders for it.
when i started as a painter i thought the sanding would drive me crazy. now im that used to it, its just like any other job.
So hows it looking? no shinny areas? did you smooth out the bitten-up areas in the guards?