How to seal an old petrol tank

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Q. One of my chums yesterday told me he was getting a new tank for his old Bonneville.

When I asked why, he said because he put Petseal in the tank and now that petrol has a higher percentage of ethanol in it that is dissolving the Petseal so that it’s all gone soft and bubbling up, gumming up the carbs.

I have Petseal in my old Ariel so it’s worrying me because the missus wants it out of the dining room and back on the road.
Paul49, MCN forums

A. Ethanol is causing problems on metal tanks sealed with standard Petseal and some glass fibre tanks.

You need to get the Petseal out and replace it with an ethanol-proof novolac-based sealant (there are a few out there, American fuel has contained high concentrations of ethanol for a long time).

The trouble is that the acetone or cellulose based solvents that you require in order to remove the previous sealant don’t do your paintwork any good, and because your tank has holes in it (that’s why you Petsealed it) the solvent could wreck your paint.

You’ll need about a litre of solvent which you can get from auto-spares shops. Swill it around the tank and leave it for several hours while it breaks the sealant down to a polyester resin.

That can then be rinsed out with hot, soapy water. Use 2-3 litres of water and an egg cup of Fairy Liquid, as that has a lot less salt than own-brand detergents.

To avoid the sealant doing any more damage than necessary to the paint work you should shrink wrap your tank, including the filler aperture and fuel tap, then cut a cross in the filler cap hole, fold the shrinkwrap down into the hole with tinfoil over the top held in place with a rubber band.

Once you’ve rinsed it out, leave it somewhere warm for several days to completely dry out before applying an ethanol-resistant sealant like Caswell GTS1750, Petseal Ultra or Tank Seal Products Sureseal.