It seemed to basically work. Not a super-tough surface but it was a lot better than before so I went ahead and did the rest of it. Redbeard says to me, "try and mix the poly and dirt into a slurry and fill the cracks in the floor. And don't get ahead of yourself." I sort of did that, but as much dirt and dust as there was on the floor, it still wasn't enough to fill the cracks, and it seemed crazy to dig up more dirt outside for that purpose. That could still be done some time I suppose. It took three gallons @ $40 to do the whole floor. I wanted to hurry up and finish it so I could stop using electric boiler heat to warm the greenhouse floor.
I had a lot of stuff stored in the greenhouse which I had to move inside. The upside of this is that there's much less floor to sweep in the house.