I removed my exhaust to ship out to Kevin to get worked, and saw that there was a reddish/orange sealer around the header. I have never seen this used before and was wondering what it is (assuming it's just a sealer to not allow air in) and if I need to re-apply some additional whatever it is back to the header. Thanks.
It's to prevent carbon tracks or traces from blowing back through an imperfectly sealed slip fit collector. It's an old trick from a long time back, but it still works. Just use hi-temp copper or black and you'll be fine after it cures.
This is a slip fit collector. Depending on how tight the fit is, I'd probably use just a very small amount. I know for a fact that this particular one requires a rubber mallet to seat it all the way up and it won't leak at all, even without any rtv, it's got a slight taper to it and a positive seal.