So…finish on this isn’t great, but here’s what it is for what it’s worth. Note that this is on that pink foam 2ft square boards you can buy from HD for like $5, so that alters a step or two. Normal spraypaint in a rattlecan actually sort of tries to dissolve this stuff (which I used to slight advantage - what looks like a rusted-out pitted section in the background toward the bottom right was a place I left the primer thin on purpose so the next coat would dissolve in some). I’ll try to be clear which steps are for the type of finish and which were because of the base material.
The design itself was just a google search for one of the older-school logos from the 2nd game, imported into Inkscape, played around a little to widen some of the gaps etc. so it would carve with a 1/8th bit, carved all one depth. I actually did the 0.25" depth in 2 passes at 0.125" each an a travel rate of like 65…got a little ‘hair’ on the cuts but I think with foam that’s probably inevitable once the bit gunks up a bit.
Finish steps:
Step 1: torch it. Well, heat gun at least - VERY quick passes, not too close, to burn/shrink down the ‘fuzzies’ in the foam at the carving edges. Test first on a fresh panel to get a feel for it, you can pretty quickly get this foam to outgas all its bubbles and go flat in a hurry if you overdo it. This stuff is so soft you really can’t ‘sand’ on it, it takes a permanent dimple from a fingernail. Then I just hand rubbed it a bit to try and carefully break off anything remaining that was too fragile - a soft toothbrush might’ve been acceptable but I didn’t have one handy, so pinky fingernail picking gingerly like at your own nose works too.
Step 2: prime. For this foam I tried Kilz, just slapped it on pretty heavy with a disposable brush (bristle, not foam) then went back and ‘drybrushed’ to get as much excess out of nooks and crannies as I could. That step would probably be skipped on another material like MDF or whatever, might use something to seal the MDF against moisture absorption or whatnot instead (shellac, whatever). Because Kilz is white I would’ve preferred not to use it, but learned from an earlier carve what happens when rattlecan paint hits the foam directly the hard way. 
Step 3: Base color. Any old rattlecan of flat black. I have no idea what brand I used, been in the garage for decades. Light passes, different directions to get the vertical carved sides but not too heavily, never want it to look glossy wet as you go or it penetrates the primer - did do on purpose in a couple places for the melted-down pitted effect.
Step 4: Iron paint. The stuff I use is by Modern Masters, “Reactive Iron Paint”. Its a pretty dark grey-black and not terribly pleasant to work with. Stinks and it’s very ‘gritty’ - almost impossible to mix smooth, but the sort of sandy texture goes along with the rusting to follow. This gets brushed on somewhat thick again with another disposable bristle brush, then try to work out excess same as step 2, and tried to lay down somewhat uniform brush direction as a final combing of the face (the stuff just does not smooth out evenly…think of sand in snot and that’s about the texture). The instructions say do one coat, wait an hour, then do another, but as I already had a black basecoat I just did the one.
Step 5 is the reactivity. Important part here is the iron paint from step 4 should still be damp!! Probably many different reaction agents could be used - watered down muratic acid (like for pools), vinegar, heck probably even lemon juice would work. I’m lazy though and bought a premixed reaction agent that normally does a Japanese Brown verdigris on smooth finished iron if you put it on hot metal and let it dry almost immediately, then clearcoat. Not clearcoating lets the verdigris finish turn to rust pretty quickly which is what I was after anyway for this piece. So I just doused it good (spray bottle) and let it sit outside overnight (moderate temps low humidity, nice Texas fall weather starting here) to dry. The mix I used is from a company called Sculpt Noveau. (I also have some copper nitrate and other green/blue verdigris finishes from them to try on the bronze paints.)
Last step, clearcoat. The clearcoat I used is also from Sculpt Noveau and is their ‘matte’ clearcoat in a rattle-can - looking at the labeling and judging from the smell though…it’s just a spray lacquer of some sort. Again, this could likely dissolve into the foam if I didn’t already have the other gooped layers on it. A couple layers, quick passes, really just enough to hopefully stop further reaction (not that I’d mind) and ‘glue down’ the loose particulate rust finish against handling damage a bit.
It’s up on my office wall now at work. I’m a tad bummed no one has noticed yet so I can brag on it here… 
Some links for the paint and finishes:
http://www.sculptnouveau.com/Details.cfm?ProdID=34
http://www.modernmasters.com/products.asp?mode=group&gid=68
Alternate reactive metal paint company: Sophisticated Finishes has come up in web searches.
FYI I bought all these to refinish my new computer case build to look like aged metal; was intending to blend the iron and antique bronze paints together and hit it up to get hopefully a mixture of green, brown, tan, rust effect streaks on it. Haven’t had the guts yet since it’s a CaseLabs $500 case, but that’s fairly high on the project list for this fall/winter.
I had to cop that it was foam to keep the cubicle police from getting wound up.
