Flex Seal isn't for use on pipes under pressure. I can't even imagine the future damage that could result in.
What Does Flex Seal Spray Not Work On?We do not recommend using Flex Seal on foam. Some foams may react poorly with Flex Seal, and may warp or dissolve. We also do not recommend using it in situations where it is subjected to high heat or pressure (such as engine radiators) or gas tanks. We do not recommend the use of Flex Seal spray on vinyl. Always test before use.
LOL, I was being sarcastic...