I filled up the fuel tanks with water to leak test them. One of them leaked through the back of the fuel quantity sender. No problem I will replace them both – they have been lying around a while. The other leaked out of the outboard rear baffle seal with the rib. Nasty.
There is a thread on Van’s Airforce – http://www.vansairforce.com/community/showthread.php?t=95262&highlight=leak&page=2 dealing with prophylaxis – rather fiddly and more likely to cause problems IMHO.
However it does have some input from pros and alludes to industry practice. There are products that do this task and apparently an A and B class sealant. B is a top coat apparently. The Tech specs are here and time to do some research I guess. http://www.ppg.com/coatings/aerospace/sealants/sealantsproducts/Pages/FuelTank_StdTech.aspx