The seal with the O-ring doesn't need anything to seal it. The felt filters that CARF provide are excellent and I have used them for years with no issues. The CARF tanks aren't as fragile as they look to be. I used to worry about fiberglass tanks verses kevlar. In the early days when the jets would carry propane on board, the kevlar tanks were to help reduce the chances of a huge fire when wrecked. Now that propane isn't used on board by most, the chances of fire have been reduced. I've seen guys used Sullivan and Dubro tanks, and even pop bottles. I thought they were crazy, until it was pointed out to go drop a 2L soda and watch what happens. It may blow the top from the carbonation, but the plastic isn't going to tear. The strength of the tank is more for the wreck, then for daily usage. Now, I noticed that you have said that you are going to make this jet a twin. If this is your first jet, I am assuming this since you are asking common questions about the fuel tanks, a twin engine jet isn't a good place to start. One of the biggest issues that is with a twin, is keeping one engine from reading the other engine's rpms. You need to rotate the engines. If this is your first jet, I would highly recommend a single engine one. Good luck.