This seems to be a dealer decision. If your local shop tells you that Triumph refuses to replace the tank, try another dealer.
As a last resort, drain the tank, take it off the bike, set it as upside down as you can manage and use steel wool to remove the paint. Wash out the tank to make sure no loose paint is in there. It's also a good idea to remove the petcock (careful with the gasket!) and make sure no paint bits are clinging to the strainer.
