Some help here http://www.thumperta...-peel-off-tank/
Some folks have coated their tanks with THIS , yes it's approved for plastic tanks. It may help keep fuel vapors from leeching out and bubbling up the decals. You still have to do some good prep work.
Others have hit their tanks with bedliner coating :> for a utilitarian look.
I cleaned part of my tank really well and put a decal on it. It bubbled up after a few weeks just sitting in the garage.
Tank cost to much for me to screw around with it and a torch.