If the tanks just full of crap and surface rust inside, Go to a garden centre and buy a bag of smooth garden stones, or ball bearings, something along those lines, sling them in your fuel tank and shake it around with a friend, turn it over and do the same, this will smash the rust off the inside of the tank, then you want to get a POR-15 fuel tank repair kit, it will clean it inside and re-seal it, will be like brand new, paint the outside in hammerite or something similar while your at it to protect it.