Why doesn't anyone use this to seal their floors?...

[nikwax]

how is the black goop any different with respect to trapping water? It's surely not a membrane that passes water out from the top but not in from the bottom? If the black goop is waterproof, then it will trap water from the top side, yes?

[nikwax]

I used Thompson's water seal..like Jack sez, it don't rain up.

it sure does rain up on in a motor vehicle...hydrostatic pressure will drive water up and in. Ask the owner of any early Chevy Blazer, the wheel wells leaked into the foot wells.

[Steve_Cox]

how is the black goop any different with respect to trapping water? It's surely not a membrane that passes water out from the top but not in from the bottom? If the black goop is waterproof, then it will trap water from the top side, yes?

"Black Goop" leaves no void between it and the plywood to fill with water, as the plastic did in the 3 examples I mentioned. I'm really hoping you use the reinforced plastic sheet since you bought it. Just 'cause I wouldn't doesn't mean you shouldn't. I say go for it.