Frank,
if you are thinking of a sealant for the petrol tank, I recommend POR15 tank sealant from Frost at
http://www.frost.co.uk/productList.asp?catID=55 Scroll down to the bttom of the page for the Motorcycle kit. The kit includes a cleaner, a rust converter and a sealer and if you follow the instructions it works great. I did mine last winter - the bottom of the tank was alarmingly rusty.
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