Ah, I see it now. Thanks for the explanation.
Many options for sealing that bottom corner.
1. "The Mix", various ratios of oil based poly and mineral spirits.
2. Straight or water thinned TB2; or a coat of each (water thinned first for penetration).
3. CPES (expensive), either multiple coats or covered with an epoxy coat.
4. Straight epoxy.
5. Epoxy and glass composite.
6. Canvas and glue or paint (foamie style).
From what I gather the CPES is rather dear and many folks feel that there is no need to bother if you are using regular epoxy over the top.
On TPCE I used TB2 thinned 1 part water to 2 parts TB2 on the underside of my floor, followed by a coat of straight TB2 and I have to tell you that it seems to be fully adequate. Before I am done I will probably add a coat of exterior latex paint and never think twice about it.
No black goop? Heresay? Maybe, but the TB2 set up hard as epoxy and has not scratched or flaked while dragging my walls over it and using it as a work surface.