You can repair a small hole using the repair kit as stated in the fuel tank manual. It is very much like repairing a car tyre.
You can refer to B707 manual or other aircraft having a bag tank for the spec. of the adhesive etc. The AMM has the reference of the vendor selling it.
I have never known of a paint to seal the porosity of a bag tank. Also the adhesive paint would peel off and clog the fuel system.
However, we used to apply a layer of the red running Pro-Seal on top of PRC 1422 B1/2 sealant which was used to repair a SOLID integral tank. PRC 1422 is replaced by a quick setting sealant (PRC 1826/? B1/2) now. PRC is not the same adhesive for bag tank repair.