If ethanol is a concern you can coat the inside of the tank with something like Caswell before painting it...
I've been a professional bike painter for over 20 years and please do yourself a favour: DO NOT EVER COAT THE INSIDE OF A PLASTIC TANK !! I've never seen or heard of swelling on a plastic Triumph fuel tank in all those years. I've painted loads of them and they're OK. If other bike brands have swelling problems it's simply not
Start by emptying the tank and let the inside dry. Remove the fuel cap and the fuel pump (if you wish, or mask it with tape) and thoroughly degrease the paint. Dirt, grease and polish will be deep in the scratches so take your time for this. Later on while sanding it's best to degrease the tank again from time to time.
The decals will be under a layer of clear coat so first the clear coat must be sanded off gently (dry P400 or wet P800). Gently pull off the decal, if you want you can warm them a bit by using a hair dryer. If there's sticky stuff remaining you can use the degreaser to soften it, then just rub it off with your thumb.
Sand the tank with the above mentioned grades and sand out all imperfections. To maintain the curvy shape of the tank you can use a small squeegee so the sandpaper follows the surface correctly. This way you won't get flat spots in the paint. For difficult to reach spots you can use scotch brite (grey).
The spots where you can see the plastic of the tank must be primed with a plastic primer. Now you can apply the filler, preferably a 2 packs filler. Let dry thoroughly, sand with P500/600 dry or wet P1000 and start applying the colors. Finish with a 2 packs clear coat.
Ethanol should be no problem, just keep the fuel inside the tank and wipe off after you filled up.
If you have any questions just let me know!