I have used pro-seal.. It's an aviation/aerospace fuel tank sealer.. The old way was two cans, part A and part B. It still comes that way if you want and is used mostly by home build aircraft. A&Ps now get a single tube with bolth parts separated until you mix it in the tube and put it in a gun.
If it absolutely positively has to stay sealed no matter what it is then clean it with acetone and a soft cloth. Wipe it with a tack cloth and apply tank sealer..
I have used it to seal everything from helicopter gear boxes to composite fighter jet fuel tanks. I saw how good aviation tank sealers were and started using them on auto, bike, snowmobile.. you name it for a gasket maker.. It never fails..
A local aircraft parts place should have it!
All about tank sealer.
http://www.vansairforce.net/articles/tank_sealant.pdf