Dropped my original exhaust manifolds to a friend's shop and he sent them for a few rounds in his block tumbler using some steel shot. Gotta say the exhaust manifolds came out looking great. When things warm up here, I'll spray them with a high temp clear to keep them looking this nice. Here's the before and after shots and some pics of the tumbler he uses for cast iron parts cleaning.
Not sure who all has access to a tumbler in their area, but I know MacNeish also does this for people.
How they looked before being tumbled.
A look inside his tumbler as he just finished cleaning a block.
The steel shot that he uses for cast iron clean up.
And the exhaust manifolds after being cleaned up.