Two versions of "the same epoxy resin" tested in a plastic container which had been sanded to help the resin key to the surface. This was done over about a two hour period so the resin had still not set but had started to gel off and should not slump any further. The version on the left is the resin I use to seal fuel tanks and it is also used in the adhesive as its base resin. The version on the right is another companies product that is sold to seal fuel tanks too Both have had a black pigment added just to help show how the resin can slump as it cures.

The second resin is what most people buy and pour inside their tanks to help seal it and as you can see over the two hours this time lapse took it may look like its coverage is good but as the resin starts to catalyse the exothermic reaction starts and it thins out the resin so it starts to flow. To have any effect the second resin needs you to keep rotating the fuel tank to be sure it stays covered and keep rotating for at least 2 hours slowly and continually.

The first resin is just applied and left to cure and will not move as it sets. But then this is one of the reasons it is so expensive. It has thickening agents but it also has a colder cure so there is less exothermic reaction which could thin out the resin and allow it to start to flow like the resin on the right.