I guess the real answer is "it depends"...
I mean if you have an old Levinson or Krell, then you might want to use some sort of paint (or powdercoat or even anodizing) that would stand up to extreme temps. Older tube amps migh have an issue with "regular" paint too--some of those old ARC units get downright toasty, and CAN burn your fingers when you run them hard...
But if you've got a Carver or a Sunfire, you could probably touch them up with model paint and they would be OK.
If they are REALLY scratched up, you might consider stripping the scratched panels and having them powder coated. I've seen some vintage amp restoration projects that were powder coated and they looked STELLAR.
But if it's the faceplate that needs touched up, you've got to worry about the branding and control markings. If you strip that off, you're going to need to have it printed back on somehow after refinishing, and that's going to mean having a silkscreen made, and finding someone who is willing to do a one-off silkscreen print on something weird like an amplifier faceplate, which can get pricey--not because it's difficult (it's not), but because it is a HUGE annoyance for the average silkscreen shop to do one-off work. They will quote you 3-5 times their standard per-piece rate, just to try and make you stop bothering them...
Tell us what sort of amp you have, and maybe post some pics of the scratched areas, and then we can better assess what your best options are.
--Richard
