A couple things I've picked up from Stormtrooper circles over the years that have also proved useful on other projects...
• If the scratches are minor enough, consider investing in a
Novus plastic polish kit. The different numbers on the bottles indicate the depth/severity of scratch they're intended for, and you can fill and smooth over some fairly significant scrapes and scratches. This is also good for ongoing maintenance.
• The ground-up ABS dissolved in acetone approach is probably best for adding plastic back on. (Personal observation: I flailed a bit trying to describe what I meant there. There's no useful English term, but I know the French
chanteau from my clothing-design studies. We need a generally-agreed-upon term for "bit added on to extend something".) In addition to techniques for grinding up some of your ABS trimmings and the right ratio to use, I've also seen people recommend (and can attest to) using pieces of a thin undershirt-type white T-shirt as a sort-of fiberglass-like backer to reinforce the join line. Since I don't know if th etutorial(s) you've found mention anything like that, I wanted to toss it in.
--Jonah