As usual, I've probably over complicated this. So if I were going to paint a plastic Tec tank, I'd prep both top and bottom steel mounts, and paint them with a rattle can etching primer. I'd mount them to the tank bedded in a light layer of epoxy so it squeezed out onto the tank. I'd fillet those edges with an acetone rag. Acetone the rest of the tank and let the epoxy cure.
Then I'd spray some Nason (from Napa) Chroma seal white 7710S. This is a sealer coat, two part. After it flashed, I'd shoot it with two medium coats of 435-82 Urethane white.
That would probably be enough to prevent damage from fuel, and keep fuel spills and seeps from invading the spaces between the mount and tank.
For extra fuel resistance, two medium coats of 465 Urethane clear would ensure fuel didn't eat the paint. The downside is that the tank would be more glossy than the engine and might not look right.
That's my answer on how I'd do it, and essentially how I paint metal where fuels, oils, and solvents can be a problem. I know others have success with other methods, but I have had issues skimping on paint on tanks.
Any one who's stripped a Briggs or Tec metal tank has noted how veracious the paint they used was. I believe it was an epoxy-based paint however. :shrug: