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There are ways to find out what plastic is what. You can get plastic welding rods from welding places that sell plastic weld guns or bunnings ebay etc. You need to take the top layer of plastic off of what you are welding aswell as the rod you can use scaper or die grunder with deburring tool. You then heat both rod and surface as you start laying a line with the rod weld a couple of cm and let cool. If it pulls off uts not compatable if it doesnt then ut is. I think gas tanks are usually made from PE polyethylene. And i would be careful what plastics you are welding as some are extremely toxic. Nylon for example.
We make petrol tanks out of PE i havent heard of PTFE or PET-G can you please tell me what they are?