Removing undercoating from gas tank??

Fill it full of water and take your propane torch or heat gun and putty knife to it.Dump it out,swish some methyl hydrate (alcohol) around a couple times and you're good to go.No explosive issues.No water left over.I know your post is 3 mos.old and you've probably spent some bucks on a new tank by now but somebody else might be able to use this info.
If you can,sneak it down to your local carwash first and pressure-wash the heck out of it .(put it in the box of your pick-up and pretend you're cleaning the truck box?)Probably won't work in CA but still works in some parts of Canada.
Hope I've helped in some way.
Cheers,