Locate leaks in the fuel bladders (most planes have several) by wiping away any oil on the surface layer, testing the engine and checking to see if any new oil appears. Examine the top of the fuel bladders first, because faults are more likely to occur at the top, where there is less fuel and more exposure to the sun. Wipe leaked oil away using a dry cloth.
Unscrew the screws and bolts holding together the outer panels of the wing cavity using a screwdriver and wrench. Reveal the fuel bladders, which are self-contained units usually suspended from hooks inside the wings. Take out the fuel bladders carefully and slowly by unclipping the clips and connectors with your hands. Use pliers to unclip stiff clips.
Identify which type of fuel bladder construction your aircraft has by looking at the materials it has been made from and any documentation that came with the aircraft. Look for two or more layers of rubberized material, the inner of which contains the fuel and is non-porous, and the outer of which is more durable. Look for nitrile rubber or urethane constructions. According to Floats and Fuel Cells, a supplier of fuel bladders, urethane constructions cannot usually be repaired easily, but nitrile-based, crash-resistant versions often can.
Clean the surfaces you wish to work on using a cloth and a little solvent to remove dust and facilitate adherence between each material. Cover any holes in the fuel bladder which may be causing a leak using newly purchased aircraft fuel bladder rubber, which is vulcanized for strength. Position new rubber coverings internally if you have enough access, as internal placements are usually more successful than external placements. Fix them on the outside if you do not have access as the repair will still work this way, but may not last as long as an internally located repair.
Replace thin, worn pieces of metal in the outer wing casing using new metal sheeting which matches the original metalwork. Fix the replacement metal sheets to the wing casing using heat-resistant sealant, which has strong adhesive properties. Try to locate the replacement sheets of metal internally if you can, as with the rubber bladder patches. Fix them to the outer surfaces if you do not have access to the inner surfaces.
Coat the patches of new material with fuel-resistant sealant, which works with metal and rubber. Seal new joints using the same pliable sealing compound, such as "Proseal", "Gore Skyflex" or "Kemfast" sealant, which are often sold unmixed as a two-part product. You will need three syringes (available at veterinary outlets) with volumes of 5 cubic centimeter, 20 cubic centimeter and 100 cubic centimeter capacities, according to Experimental Aircraft. Use the large syringe to measure 80 cubic centimeters of sealant part A (the base) and the medium syringe to measure out 1cubic centimeter of part B (the catalyst). Always mix part A and B in a 10 to 1 ratio. Mix the two components well on a flat plate. Cut the end off a third syringe using a razor blade to create a diameter of around 1/4 inch. Load the mixture into the application syringe. Apply mixed sealant using the cut syringe to the repaired bladders and wing casings. Spread it out evenly using a paintbrush.
Replace the entire fuel bladder if a repair does not work or the faults are too extensive to be resolved. Set aside between 8 and 10 hours for installation of a new fuel bladder.
