I have a 1989 DT200R sitting under the house that I used to race back when it was new. After one particularly bad stack, the bike sat waiting for repairs for a while and the tank developed really bad rust. I ended up sticking a few handfuls of bolts and nuts in the tank, wrapped the tank in bubble wrap, wedged it in a cement mixer then ran the mixer for several hours. That got rid of the worst of the rust. I then used a Kreem tank liner kit. The first part is an etcher which dissolved all the rust and etched the metal. The second part was the liner. After pouring it in you have to continually rotate the tank around to get an even coating and drain off the excess before it gets to viscous. On tanks like the NSR that have a lip you can only really drain the excess through the hole where the fuel petcock attaches. The liner lifts paint better than any paint stripper I've ever used as well. I can't vouch for ethanol based fuels, but for fuels in Australia the liner seems to work fine. Mine is going on 24 years now and is still as good as the day I applied it.