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I wanted to make very sure that my fuel senders were sealed before putting the tank in the wing and then filling it with fuel in the future!
The builders guide says the screws should have fiber washers under the head for sealing. These are no longer provided in the supplied VDO install kit. It sounds like a very good idea to have a fiber washer to help seal or a copper washer - like the seal washer used in banjo fuel fittings. I am also using the polysifide rubber sealant.
After sealing, here is a way to test for leakage. Get a shop vac, leaf blower or low pressure / high volume air source from a HVLP sprayer. Use duct tape to connect this to the fuel tank vent tube. Seal the pipe threads and fuel cap with duct tape - no need to completely seal these areas. After the sender sealant cures, turn on the blower to lightly pressurize the tank with air. Fill a spray bottle with water and a few drops of dish detergent, mix well and spray on the outside of the sender. If you see any foam bubbles forming, you need to re-seal. This is the approved method for checking a low pressure gas line for leaks - it should work great on your fuel tank.
Happy building, Larry Zepp Zodiac 650 B builder