Pour a quart of WD-40 into the tank, along with a handful of 3⁄8 nuts. Close off the filler neck with the cap or a plug and start shaking that tank like you ain’t got no brains, as Bruce Springsteen might say. In fact you might want to put on some earphones and turn your favorite tune up to 10 to enhance the experience. After about 15 minutes or longer, if the rust warrants it, drain the tank, remove the hardware and flush it with a little fresh gas or brake wash. You can finish the job by coating the tank with a plastic sealant like Kreme, or if it was just surface rust, simply put it back in service. Whatever you do, I’d recommend replacing the fuel filter or adding one if none is present to catch errant rust particles before putting the bike back on the road.