Drop the tank and use steel shot inside the tank as an abrasive and what I did was suspend it in a pick up truck bed and drove around the city sreets about 6-7 miles and then flipped over and repeat. You and a buddy can just shake it to instead though. All the crusty rust chunks came out and the inside is shiny again. Use a telescoping magnet to gat all the shot out. Vid I saw they used gravel...but that seemed like a major PITA fishing out of an E30 tank so I got steel shot from Wholesale tool cheap like $2 a bag.