N/A guys are fine with this(coating inside). Like kevin has said, it needs to be cleaned, degassed, sandblasted and then coated after a final particle removal process. The insides obviously don't get the sandblasting/beadblasting done other than the ends that are reachable by the ends. This is the most important part of the process. The headers that are for N/A cars get the inside as an upgrade or bonus if this is the poilcy of the coater, so they would think they were doing you a favor if it gets done. And the guy coating is probably some guy getting just over minimum wage in some cases, so he may not have the gearhead smarts to figure it all out himself. It's really important that you make it known it should be uncoated. Big red letters on a sticker on the part when you ship it. If it were possable to sandblast the inside of the part, I think the flaking issue would be solved. The coating does keep the part cooler on the outside in comparison to no coating, and would be even cooler if it were coaten inside as well.
1977 911s. it's cool