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.

Using Proseal

Proseal is a two part fuel tank sealant mixed together just in time and in correct quantities to do the job at hand. It can be messy job but if handled properly, easy to work with.

By following a few simple rules, it will be a lot easier to use Proseal. Lets establish some need to know items before we start:

• It's cure rate is thermo exponential, ie the temperature effects the cure rate
• You can mix smaller quantities by volume and have the same cure rates and results
• Like most resins and epoxies you will use a catalyst to cure the product
• Obtain syringes of different sizes (5 cc, 20 cc and 100 cc types) to apply the Proseal when it is mixed
• Get latex gloves, some raiser blades, paper towels and MEK or thinner for cleanup

When mixing Proseal in smaller amounts up to 40 or 80 cc by volume we use large syringes available at most agricultural and veterinary outlets. This process insures that the Proseal is accurately measured and easy to handle and apply. You will need three syringes: two for parts A and B with the end cut off by the raiser blade (open cylinder) and the third with and opening of 1/4" for applying the mixed Proseal.

By cutting off the ends of two syringes so just an open cylinder is left on the end of the syringe it can then be used to accurately suck up measured amounts of the base and catalyst. Using the 10-1 base to catalyst ratio 2cc of catalyst for 20cc of base. Two syringes must be used to prevent cross contamination of the products A and B components.

After the measured components are mixed thoroughly on a flat plate for at least one full minute they are then loaded in to the plunger end of a third so called application syringe. This is usually done with a oil artists knife or spatula. Once it is in the application syringe you can apply it exactly where you need it and just the right amount of it. A small 1" diameter fine felt paint roller or heavy brush can then be used to uniformly spread it on surfaces. Buying the 3" long by 1" diameter paint rollers and cutting them in to 3 separate 1" X 1" paint rollers works well. Use a small tube to take up the slack in the paint roller reel. You can rapidly measure, mix, and apply Proseal by doing it this way and there will be hardly and waste product or expense to contend with after.

Cure rates

The cure rate of Proseal increases by a factor of 2 for every 10°F . above 70° and is retarded by the same factor for every 10°F degrees below 70° This means a cure of 12 hrs at 70° will occur in only 6 hrs at 80° and only 3hrs at 90°. The same process on the cold end of the scale means Proseal will take about 24 hrs to cure at 60° and 48 hrs or longer at 50°. 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.

Larger gaps
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 angle hair (long strand fiberglass wool) 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.

Note
Do not mix angle 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

Some tips

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.

If you want to use a sloshing compound in your tanks, make sure it can handle the alcohol which is present in Mogas these days. Contact the manufacturer of the compound to confirm this. Rotax engine owners with metal fuel tanks in their aircraft should be aware of this.

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 this over the rivets. Bubbles will show a leak.