Ive seen on the Honda Twins site that they are useing Bar Keepers Friend powder mixed with Water to eat rust from metal, doent hurt the metal just eats rust. after cleaning by any method i would recomend sealing the tank. I personally use Casswell tank sealer, have had nothing but good results from it.
I have used old carbide inserts from work with purple power to cut the big stuff off. I've got one right now that has some pit holes that need attention after the insert trick. I cut two large handfulls of chunks out of it.