
Originally Posted by
zoama585
What abrasive would you recommend for rusty cast iron exhaust manifolds that will be painted afterward ?
Honestly, I'm thinking the coal slag. Tell you what, I happen to have a rusty cast iron manifold off a '67 327, I'll throw it in the cabinet when I go out to the shop tomorrow and blast part of it, snap you a nice pic (will do it with a real camera if the iPhone camera doesn't show it well enough).
If not slag, I'd probably opt for something around a 70-80 aluminum oxide, blast the hell out of it, give it something like a TSP scrubbing and then hit it with some 2k* VHT unless you want to spend the money on a fancier high temp coating.
Last one I did I used that obsidian on, was for a '68 250 chevy, stripped it down to where it looked like it just came out of the casting sand.
The pieces I was running today with the slag were heavy sheet steel, probably 10ga or so, and some maybe 16-18 ga (electrical box from the mill) no distortion, but a lot of tooth left. I also ran a cast iron piece from the mill, horizontal table stop. Only thing it had issues with was the black filler they used originally, but then again I've had issues with that stuff with super strong stripper, knotted stainless wheels on a 4 1/2 inch grinder and 40 grit die grinder rolocs. I wish I could buy that stuff in the can, the stuff is damn near bullet-proof.
Oh, and I'm running my PSI up around 100-110, right around the max for several of my guns and enough over the max for one that it blew an o-ring 
Now, here's the counter-intuitive part... you may want to go with a finer slag, the 30-60 rather than the 20-40, sand cast iron has lots of nooks and crannies, the finer abrasive, yet still with the sharp edges, will do a quick job. glass bead in the 80 range would do it, but would take a lot longer, leave a smoother finish. Whatever you ultimately use, be sure to clean it really well afterwards or you'll trap residue under the paint and create weak spots or little exploding spots when the media expands or thermal shocks. COE of glass vs iron is way different.
Trip Bauer
Former USN HT
Everlast 200DX New Model
Hobart Handler 125 MIG
Van Norman #12
Atlas 12" engine lathe
'98 RoadKing - 84 Ironhead - 59 Ironhead