flex and seal plumbing repair

No, but I have used cheapo Homedepot fiberglass "resin" and some loose fibers. Just soak the fiberglass sheet or fibers in the epoxy and smear/wrap/slather on the cleaned and sanded crack. Marine epoxy would be better if you think the pipe might move or wiggle as Marine is more flexible.

Hint: while a simple, loose cotton face drape might protect you from the world's most deadly virus ever, i would use a canister respirator mask when working with epoxy - also available at HD.

Fiberglass is not ideal for sticking to plastic. Using a better, real epoxy, resin would be better, but more expensive.
All you are doing is stopping water leaking from a few inches of pressure, so my repairs have worked.
Also, JB weld for plastic might be enough too.

VHB tape should work on the pipe, but not at a junction. VHB tape does not like corners or ridges, just flat surfaces such as a pipe.

Eternabond won't hold on waste tanks.

I have a leak on my grey water outlet pipe. It rubs on a bolt near the wheel axels - poor design, surprising I know. Anyway, I had some flex tape from a pool leak that I tried flex tape. Flex tape did not fix either leak. All over youtube people fix leaks with flex tape. I found it doesn't work.

Hi Don,
I once cracked my black tank pipe behind the gate valve while backing down a twisting, vegetation overgrown driveway. I hit the dump pipe on a stump sticking out.

I emptied the tank, parked it so the drain was high and got under there and very thoroughly cleaned the area of all dirt etc, especially around the crack in the pipe and fitting.

I used Blueskin, which is a building construction waterproofing membrane, peel and stick similar in properties to Eternabond. I used a heat gun to make sure everything was toasty warm after putting it on.

After covering the offending areas it never leaked again.