My experience is too much heat makes it gray and chalky. 65 to 75 amps sounds about right but I very rarely hit wide open, I don't back purge exhaust either unless it is being polished. It's not just the amount of heat but how long you put the heat in. That's why fitup is so important, if you are trying to "bridge" a gap that requires some time and that can burn the color out of it. That's been my experience.
Here's a secret, I don't even use filler rod on the seams of unpolished exhaust. SShhh.....the welding nazis get mad when they hear that, but that's the way I do it and I have never had a problem.
Headers are another story, I always backpurge.......well not really. I use a local shop that welds sanitary stuff all day, everyday, they do it so cheap its not worth my time. Same with polished exhaust, I tack it and drop it off, and it comes back polished
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Donny
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