I used 2 quarts on my 27' two coats with floor on. I brushed on. Pretty sure the dogs and I had at lest a quart on us. Great stuff will not come off your skin. I suggest long plastic gloves the ones they use to pull a calf and a face shield. Scratch that. A full body suit. The stuff soaks right thru cotton onto your skin.
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Master of the Tin-Can and thats not saying much
Don't buy it in gallons--quarts are less expensive in the long run, since you can ruin an open can very easily with a not so perfect seal.
I have two experiences:
1. Took two weeks to get the skin off (paint does not come off. period.) Yes, gloves and cheap T-shirts at a minimum.
2. Since you're buying in quarts, get half gray/green/brown or any other color than black and the other half black. If you're brushing on, you can't tell the wet from the dry if you do two coats of black. POR-15 has excellent leveling qualities (very smooth, even when brushed) and shiny.
I also agree that 5 quarts is too much. Not more than 4 and more likely 3. Guess it depends on your brushing style.