Greenpatch 421 is better than Satanite
I've used both Satanite and Greenpatch 421 for sealing Inswool/Kaowool fibers when making forges. I will say with certainty that Greenpatch 421 is far superior for the task. When applying the first coating of Santanite it cracks a lot when it dries. The Greenpatch cracks much less and almost not at all. Both products need more than one coat but the Greenpatch 421 can also be used to patch your forge in up to 3" sections.