Q: I want to coat some bunk beds, and need to know how much finish to buy. When finish makers state that the coverage is X square feet per gallon, and that one should use at least three coats, does this mean that the gallon will cover X square feet three times or just once?
A: Just once. A claim of 400 square feet of coverage per gallon means there’s enough to cover 400 square feet once. For three coats of 400-square-feet coverage, you will need three gallons. However, coverage numbers are only average guidelines, and the amount of coverage you get can vary greatly, depending on three major factors.
The first is the substrate. A very porous piece of wood will absorb, and consequently use up, more finish than a coat applied to a denser wood or to an already sealed surface. Thus, when working with porous woods, odds are good that on the first coat, you will use more finish and get a lower area coverage rate than on the second coat.
The second variable is your application method. Brushing wastes almost no finish: nearly 100 percent of the coating in the can will end up on the wood. However, spraying wastes anywhere from 35 percent to 75 percent of the finish, resulting in far lower coverage rates. The third variable is you. What you call “one coat” may be much thinner or thicker than what the next guy calls “one coat.” Thus, your coverage per gallon may be much more or less than the company estimates on its can.