Sorry Rod I've never lined any plastic tanks, and would think that it would not be successful with epoxy, because the HDPE used for plastic motorbike fuel tanks has very low stiffness, so any impact would load up and crack a stiff lining like epoxy
The epoxy I use is low viscosity (made for brushing) so it can be spread inside fibreglass tanks by moving the tank around and letting gravity move the wet resin. The inside has to be completely degreased so the epoxy sticks to the polyester resin and any exposed glass. I wash the tanks out with hot water with dishwashing liquid, then rinse with hot clean water, then dry the tank fully, then do a rinse with acetone, then mask the places I don't want the resin to go using plasticine. You need to mix up excess epoxy, coat the inside, then let the excess drain out before it starts to thicken. I experimented with the resin and hardener mix to get the right ratio to provide at least 30 minutes before it started to set. I've used two different brands of brushing epoxy, both bought from a local boat shop. I think the first was Epiglass and the others have been West Systems. From memory It costs about $50 in resin to treat a tank.
It's not just for protection of the original polyester resin that I line them, but most of the 1970s Spanish tanks I have looked inside there is a lot of exposed glass strands and I want to make sure the tank won't leak and ruin the fancy paint job that goes on later.
Back on plastic tanks, some people claim to have had success keeping stickers on them using some sort of external sealer coating before adding the stickers
relax, nothing is under control