Unknown Sealant in Tanks

Have an older set of RV-9 wings, found this dark, amber-colored sealant in the tanks. It appears as if it is covering the pro-seal. Hit it with lacquer thinner, which re-activated it, but found that rubbing gasoline did not have any effect.

Does anyone know what this stuff is? I think these wings are around twenty years old, and don't appear to have ever been wet.







Any help with identifying this stuff would be greatly appreciated. If you look carefully, you can see the Proseal in the second image, so it's not proseal that has separated with age. This is something entirely different.

-Ryan

Very little proseal. The over coating looks like it could a a common slosh compound, which would also be softened by lacquer thinner.

Tank Slosh

See Vans service bulletin SB 11-9-13.

Dick DeCramer
RV6 N500DD
Faribault, MN

It doesn't look like the slosh compound we used on certified aircraft 30-40 years ago. BUT, I would be very cautious of any unknown in a fuel tank.

Possibly Proseal PR-1197

It looks very much like Urethane topcoat such as PR-1197, and similar products used in production as a "Vapor Barrier/secondary sealer" for integral fuel tanks on Boeing products. Typically, it is brushed or sprayed over the 1422 Proseal joints on the external side of the fuel center wing tank. It is an amber colored translucent honey like consistency that allows inspection of the substructure through it when dry.

It could be the urethane overcoat, but I?m looking at the rivet tails with very little proseal and no fillets on the flanges. High likelihood the sealing failures.

Looks like the stuff I found in a pair of 9 tanks I had to rebuild. These tanks were major leakers that had been 'repaired' at least a couple times with various methods, the last attempt with this epoxy stuff that was hard, brittle & curling away from the tank in some locations. Unfortunately the only solution for this tank set was to drill the tanks apart & completely rebuild them properly.

Unknown Sealant

Has anyone used Flamemaster CS3600? This is a front runner - going to contact manufacturer to see if they think that is it.