Motorcycle gas tanks come with a coating of sealant to help protect from rusting. You need to be careful what additives you use as some will break down the sealant and get into your fuel system.
I have used this product in the past for internal maintenance of an old fuel tank http://www.por15.com/POR-15-Fuel-Tank-Repair-Kit_p_62.html
no problems with the lining after it was all done but not a quick job many hours of sloshing various fluids around in a tank and rinsing with water after each fluid
I sealed the tank and i didn't get all of the old red sealant. I couldn't. I sealed it and it looked relatively nice inside. After a fat ride I saw inside the gas looked red lol.