Coating on older DEC slides

So I have a bunch of older DEC slides (for BA11 boxes, RK05's, etc) which are rusty. No problem, I have a sand-blaster, but.... what's the grey coating on them, and how do I reproduce it once I have them clean? I saw Corey Cohen's really wonderful presentation at VCFE about restoring old computers, and afterwards I asked him if he knew anything, and he said that (from my description - alas, I didn't think to show him one in the display area) it sounded like a powder coat thing, and that people who do brake calipers can do power coating. However, when I went to my car guy for pointers to local people who do brake calipers, he looked at the coating, scraped at it a bit with a knife, and said it didn't look like powder coat to him. (Although maybe this is a very old powder coat, and he's only used to the newer stuff.) He reckoned it was just grey paint {visualize dubious-looking Noel - it sure doesn't look like paint to me}. So, what _is_ that grey coating - and, more important, how (if at all) can it be reproduced these days? Noel