An introduction to tank sealant

As part of the process of sealing my fuel tanks, I started researching fuel tank sealants in order to get as good an understanding of them as I could. I came up with a lot of interesting (at least to me) information and since the forums and newsgroups are always full of questions regarding tank sealants, I thought I would put together a little article describing the results of my research in hopes that it might help others when they get to this point in their projects.
Although we tend refer to all tank sealants as “proseal”, Pro-Seal is actually the trade name for a line of aerospace sealants manufactured by the PRC-DeSoto division of PPG Industries (now PPG Aerospace). As there are other manufacturers’s products incommon use in home built aircraft, I’ll refer generically to tank sealant or just sealant in order to avoid any confusion. There are a number of variations of tank sealants; however the tank sealants that are commonly used in home built aircraft are 2-part, manganese dioxide cured, polysulfide sealants designed to meet the MIL-S-8802F Type II (now AMS-8802) specification. This specification describes the characteristics and properties of the sealant, such viscosity, curing time, strength and adhesion, as well as the testing methodology used to determine if the sealant meets the specification. For those who are interested, the text of the MIL-S-8802 specification can be found at: .
As homebuilders, we are not required to use Mil spec products; however it is generally a good idea to do so where practical, as these products have been thoroughly tested and the results are well known and predictable.
General Characteristics
Tank sealant is used to seal integral fuel tanks by sealing the joints and rivets, preventing fuel leaks. They adhere well to aluminum, titanium, steel and other materials. Tank sealant cures at room temperature to a flexible rubber-like state that maintains its properties within a temperature range of -65°F to 250°F. They are generally resistant to most fuels and oils used in aviation.
More information regarding adhesion and fuel resistance can be found in the Mil Spec.
Basic Properties
Tank sealants are identified by 2 primary properties, viscosity and working time. There are 3 classes of tank sealant as defined by viscosity. Class B sealant is the paste consistency sealant that is most commonly used in sealing homebuilt aircraft tanks. Class B sealant is designed to be
applied using an extrusion gun such as a Semco or Techcon gun or by using a spatula or similar
tool, and does not sag or run on vertical surfaces. Class A sealant is a lower viscosity, pourable
sealant designed to be applied by brush. Class C sealant is an intermediate viscosity sealant
designed for brushing or rolling on faying surfaces. Class C sealants are not common in homebuilt aircraft as their application and cure times are quite long compared to Class A and B sealants. In addition to viscosity, sealants are defined by their working or application time. This is the time during which the material can be applied. Beyond this time, the material will cure sufficiently that it can no longer be worked and may not adhere properly. Working times defined in the MIL spec include ½, 1, 2, and 4 hour. 2 hour working time is the most commonly available from the normal sources. In addition to the standard sealants as defined by the current Mil spec., there are now some rapid cure sealants available that cure much more quickly than the standard sealants.