Well, I had never looked at Caswell.

But from their website . States their "Epoxy Gas Tank Sealer (PHENOL NOVOLAC EPOXY GAS TANK SEALER)" .........will work on plastic.

"Permanently Repairs Steel, Aluminum, Fiberglass and Plastic Gas Tanks" , as long as it's not made of
polyethylene.

"Bonds to almost anything except polyethylene (gas tanks are not made of polyethylene). Bonds to wood, plastic, rubber, concrete, metals."