Fuel Tank Sealant, Proseal II
Aircraft wing tanks come in different size and shapes. Some aircraft have a separate fuel tank build into the wing in the form of rubber bladders or even composite tanks built inside the wing. Cessna, Piper, RV and Murphy type aircraft have a wet wing. This means that the fuel is contained / carried in the wing itself in separate bays usually close to the wing root.
To successfully carry fuel by the wing one must make sure that this section of the wing is absolutely leak free. And as the wing is drilled and riveted (either with special tank Avex- or blind rivets) this means we need to seal this construction to keep the fuel inside. Proseal is used for this.
As with fiberglass resin, Proseal need its time to cure and proper handling with clean surfaces if the result is to be perfect. And with fuel tanks it should be.
Be prepared for a somewhat messy job here.
The cure rate of Proseal increases by a factor of 2 for every 10°F above 70°F and is retarded by the same factor for every 10°F degrees below 70° This means a cure time of 12 hours at 70°F will occur in only 6 hours at 80°F and only 3 hours at 90°F. The same process on the cold end of the scale means Proseal will take about 24 hours to cure at 60°F and 48 hours or longer at 50°F. These figures will fluctuate with mixture ratios and humidly but are a good base line to follow.
Although Proseal is a catalyst accelerated cure product. The catalyst will not self disperse with in the base as does something more familiar such as fiber glass resin catalyst. Because of this a single drop of catalyst in fiber glass resin will cause the whole base to cure even if its not stirred or mixed. This is not the case with Proseal and it must be mixed extra well to insure a uniform and consistent cure. This is not difficult to do but does require some extra effort on the users part.
If you find that the fit of some your components leaves a little to much gap for the Proseal to comfortably span you can fix this problem also.
Mix a little long strand fiberglass wool (angle hair) in to the Proseal and stir it up, emulsifying the angel hair in to the Proseal. The result is a fiberglass reinforced Proseal barrier which is very strong.
Do not mix angel hair into all of the Proseal as it does cause some loss of elasticity. The fiber glass strands although flexible are not stretchable. With a little bit of fooling around you will quickly find that Proseal is a blessing to work with and not the horrible stuff its made out to be.
In addition to sealing fuel tanks it has many other applications but its price makes them prohibitive. Many builders didn't like the idea of fiber glass work either until they found out just how easy it can be to work with once your familiar with the product and its handling.
Original text by Murphy Aircraft technical department
The shelf life of most tank sealant products is limited from six months to a year at room temperature. This means that you should only order the quantity needed and use it within that time frame.
It is essential that the tank surfaces are clean. Use either MEK (Methyl Ethyl Ketone) or Acetone or what the supplier of the sealant prescribes. Do not touch the surfaces with your fingers after cleaning. You will leave body oils and grease that will inhibit the sealant to seal properly or fail to bond properly to the surface.
If you want to use a sloshing compound in your tanks, make sure it can handle the bio-alcohol which is present in Mogas these days. Contact the manufacturer of the compound to confirm this. Rotax engine owners with composite tanks in their aircraft (especially Dyn Aero aircraft) should be aware of this.
Testing fuel tanks
Testing your fuel tank for leaks can be done by pressurising the tank, do not over do this usually 0.1 or 0.2 bar should be more than enough. Have some soap and water handy and coat this over the rivets. Bubbles will show a leak. Reinstalling these rivets or reapplying some Proseal will take care of this problem.