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Is it worth putting a water absorbing fluid like meths or IPA into the petrol tank on a regular basis , say yearly, to ensure that there is no water build up from condensation etc?. If so what is the best product.?

The best way to reduce condensation is to reduce the area it has available to form. By keeping the fuel tank filled with fuel will reduce the space available to form and therefore less condensation. Car manufacturers use plastic fuel tanks these days which help reduce condensation from forming in the tank.

I have recently purchased a second hand car (import) and I want to ensure that the tank is all clear of water. I had an experience once on an import which had water in ithe tank for some reason and I had to get it drained.
Would a litre of meths, which is hygroscopic, do any harm to a modern motor?. Thanks.

It is not uncommon, it is a practise that has been used for some years now, to add a cup full of methalated spirits to the tank when the water contamination is causing a problem (missfire). We are not aware of any side effects but on the modern car it would be better to clarify this with the manufacturer.