One is that Marelon is NPS.
The other is that I'll never use it again on a mushroom fitting. I don't know if the fitting I had was defective or there was a hidden hairline crack, but the mushroom head popped right off as I was tightening the fitting. I went to the local WM and sure enough, the NPS mushrooms in bronze fit just as well, so I used a bronze though-hull with a Marelon valve.
(BTW I used 4200 for sealant.)