After installing my first dozen sets of headers by paying customers, and half leaked like hell, I learned that the older header flanges/welds were seldom/never flat. (Newer quality headers are better)
Since 1970 I have always checked for straight, - draw filed or weld the hills and valleys the manufacturer missed on the flange, used supplied gkts, or Mr Gskt, and the vehicle left with no leaks, and very seldom ever came back with a leak.
I have yet to use Remflex, or any form of RTV after dozens of sets , and have to guarantee my work .
As with most things, preparation is key, - mating surfaces have to be flat .
