When I had to weld repair a Range Rover fuel tank I washed it well with a hot caustic soap solution to remove the petrol smell, ie the smell is the petrol vapour, then left it in the sun to cook for a week. I then connected it to the exhaust for a Triumph 1500. After around 5 minutes with exhaust gas purging through it I started welding whilst keeping the Triumph engine running to keep the fuel tank purged with inert gases. This is the sort of advice given in pre- and post-war hardback motoring books. Total overkill really, but I have had some wonderful explosions welding wax filled sills on my cars.