Grinder really isn't the tool to refinish metal with in my opinion.
Relatively hard to control, somewhat dangerous if you aren't familiar with them (or even if you are), noisy, messy, really a tool for shaping/removing metal or clean up before/after welding.
A good air palm sander like
http://www.amazon.com/dp/B005MVB3OO/?tag=atomicindus08-20 with some 36/40 grit to start is what I would recommend.
Then 80 grit and some good epoxy primer like spi, then whatever grit recommended for the paint you choose. (I just 220'd it, good enough for a tool box)
My local body/paint supplier made me up some single stage red that looks to be a dead match for snap on red when I did my kr660b and cost a little less than anything from his books.
Won't ever paint a tool box again though. I figured a couple of days of laid back working at it and $50 or so and I'd be set. Ho ho ho, not so.
Ended up being 5 or 6 days of hard work and more like $200 in material, tool boxes have a lot more surface area than I would have guessed.
Edit: Had to do some metal straightening and body work so that took up a day or two of the 5 or 6.
But I'm guessing you don't have air?
Have you tried using 36/40 grit on your da?
Otherwise maybe the chemical stripper mentioned earlier, some hassle involved there, but think you would have better luck than taking a grinder to it.