Hyme,You can seal your tank anytime.I used the ever popular Kreem,available at most bike shops.It worked but,I didn't like it.It took a long time to dry and clumped up on me.POR has a product thats sounds good and I think Eastwood has something too.I think Kreem was the only product around for years so everone used it.Also I think they all use a 3 step system,rust remover,metal etch and then the final seal.Just out of curiosity,if after 10 yrs it hasn't rusted,why do you think you need to seal it?Most tanks develop rust because they sat without gas in them.