A great way to make sure your tank is completely dry is to find a hair dryer with a nozzle that fits nicely in the fuel filler hole. Be careful with the amount of heat - low to medium works well, enough to feel the tank is warm to the touch. Let it run for while, 10-15 minutes and be sure that air is circulating out the bottom (petcock removed).
I recently did this to a tank I was prepping to coat with POR-15. It was so dry that I decided to let it sit overnight just to see if any flash rusting would occur. None did.. so I filled it with gas, put it back on the bike, and saved the POR kit for a future tank that has pin holes or more serious issues.