Scraping sealing surfaces

The exhaust manifold on one of my cars was warped, about 1 mm high in the middle. Common problem on Land Rover Td5 engines. After cleaning it up on my old VBRP mill it's still about 0,1 mm high in the middle - checked with feeler gauges and the "straight edge" that was my practice piece when I took a scraping class. I think this is due to wear and a bent table, the mill is a future project in itself and have several signs of abuse.

To further improve the manifold surface I'm thinking about scraping it. I have seen mentions of scraping gas tight seals, although I have only been concerned about sliding surfaces until now. Are there any differences in preferred depth, technique etc? I would assume so as the purpose is sealing and not oil retention?

Is the manifold cast iron or aluminum ? Scraping a "Static fit" is what I call what your talking about. Will you be using a head gasket?

When I scrape static fits, I scrape for percentage and not PPI. It's basically the same checkerboard technique but grind the blade steeper so you don't cut the normal .0002 deep cut. Could be anywhere between 10 to 20 deg negative depending on material. 60 to 90 mm radius blade tip. Then when you done scraping stone off the burr and the high spots to get 90+ percentage. Be sure to lower the area around the tapped holes a little, say .002, so when you tighten the bolts they pull the area around about 1/4 to 1/2 inch in diameter When I work in power plants and shipyards we try to get 98% and that's by eyeballing the blue. Rich