Just a few comments about swelling polymers in solvent that may help us understand a little more about what’s going on with these gas tank sealants/coatings (or any other polymeric coating for that matter). When a polymeric coating fails to dissolve in solvent there are generally only two reasons why this occurs; 1) the polymeric coating is crosslinked, and therefore will never dissolve, even in a good solvent, or 2) the solvent or solvent blend employed is not a good solvent for the polymeric coating.
I don’t want to head off too deep into the land of polymer science here, but a few comments are in order to help us appreciate the difference between a cured/crosslinked polymer and an uncrosslinked polymer. Let’s take a familiar example - Amazing Goop adhesive. This is a linear thermoplastic polyurethane solution that is delivered as a viscous solvent solution (toluene). Upon dry down a tough urethane elastomer/adhesive results. However, this resulting tough adhesive is not cured, and can be readily dissolved by re-exposing it to a “good” solvent (the one it was delivered from, or many others). As it dissolves it will swell and go back into solution resulting in the formation of a viscous solution.
Now let’s consider a curable adhesive example - Gorilla Glue, which has no solvent and is 100% adhesive. This too is a polyurethane, but this is a curable, or crosslinkable, polyurethane, i.e., it has reactive sites that cause the polymer to crosslink (x-link) upon exposure to atmospheric moisture. Once cured, the resulting rigid adhesive can not be re-dissolved, i.e., it may swell with a “good” solvent, but will never dissolve, and the solvent used for swelling will show no increase in viscosity, because no polymer is dissolving. The take home lesson here is that another means of reducing a coating’s susceptibility to solvent swelling is to x-link it.
Back to the issue at hand - vintage tank sealers and their removal. Tank coatings in general protect the tank by being impervious to motor fuels (non-polar solvents such as alkanes and toluene), and some of our trouble in recent times with tank sealers occur as the polar content of the motor fuel increases due to addition of ethers and alcohols. That said, tank coatings have historically been formulated primarily to be incompatible with non-polar solvents, but unfortunately, this was achieved by making the coatings themselves more polar. As our motor fuels trend to higher and higher polar content the old coatings are not always up to it, and become susceptible to swelling in the modern fuels.
The fact that Cotton’s tank coating can not be dissolved may suggest that the factory used a reactive coating that x-linked upon application, in which case it will not dissolve but would be swellable with a good (polar) solvent. According to Cotton’s earlier comments, the subject coating is swelling and becoming gelatinous, but not dissolving, as evidenced by no color (or solution viscosity?) in the solvent. This is classical behavior of a x-linked coating, where the polymer swells, but does not alter the solvent viscosity. However, it’s also possible that acetone may not be the best solvent for this coating. Sometimes the polymeric coating can be a block copolymer, and the solvent you choose will swell one block and not the other. This would also result in swelling, but no dissolving the coating.
Perhaps the best thing you can do at this juncture if you are still curious about dissolving the subject coating is to take the chips you have removed, and try systematically working through a range of solvent polarity to see if you can identify a suitable solvent. For such experimentation you want a relatively polar and a non-polar solvent pair. For the polar solvent you cold use acetone or an even more polar solvent would be isopropyl alcohol (be sure you are not using rubbing alcohol which is 70% isopropyl alcohol 30% water). As the non-polar component you could try toluene or mineral spirits. If I was doing it I would start with acetone and toluene. Make up solutions containing 0, 25, 50, 75, and 100% toluene in acetone, put chips into each, and seal. Let them stand, or better yet agitate them continuously, for days, then inspect to see what they look like (color leaching?, any viscosity change?, etc) and what they feel like (minor softening of the chip, major softening of the chip, etc), and it should become readily apparent whether the polar or non-polar end of your experimentation is getting you anywhere. Continue experimenting over narrower ranges of solvent proportion if you see something promising. In the end, if you have no success dissolving the coating with any solvent blend you may have to declare it a crosslinked coating. Someone’s earlier advice to use paint remover (for a prolonged period of time in the tank) was also good counsel, as this would stand a very good chance of swelling the coating.
One final note on tank coatings. POR 15 is a curable polyurethane that once well cured should be impervious to common fuels (and no I have nothing to do with POR 15, I don’t care what you seal your tank with, but I do like things to make sense, and POR 15 swelling in motor fuel doesn’t make sense to me). In the above instance where Cotton found it to be otherwise, I suspect that for whatever reason it did not cure adequately. Again, if you have any doubts about a coating’s integrity in a given fuel, create a well cured sample of the coating (to obtain test chips) and expose it to the fuel of interest to see how it fairs. I realize this shouldn’t be on us to figure out, but when things don’t work the way we expect our only resource is more experimentation.