Why the first coat is the expensive one
Bare wood is a sponge with a surface. The first coat of anything soaks into open pores, end grain and the torn fibres left by sanding, and what you are really doing is filling voids rather than building a film. The second coat has a partly sealed surface underneath it and covers far more area from the same amount of material. Somewhere around the third coat the rate settles down.
How much more the first coat takes depends on what the wood is. A quartersawn maple panel might take 30 percent more on the first pass. Red oak, with its open pores running the length of the board, can take 60 percent more. End grain, and the exposed edges of MDF, can take two or three times what a face takes and still look starved. This calculator applies that multiplier to the first coat only, which is why the per-coat figures it reports are not all the same.
Two things reduce the first-coat cost if it matters to you. Thinning the first coat 25 to 50 percent lets it penetrate rather than sit and puddle, and uses less material overall. A wash coat of dewaxed shellac, roughly a 1 pound cut, seals the surface for a few dollars and has the useful side effect of evening out blotchy species like pine, cherry and birch before a stain goes on.
The coverage figures used here
These are square feet per quart, per coat, on already-sealed wood, applied by brush. They are ranges because every published figure is a range, and the manufacturers who quote a single number are quoting the best case.
| Finish | Sq ft per quart per coat | Notes |
|---|---|---|
| Oil-based polyurethane | 90-125 | Builds fast, thick coats, the heaviest user here |
| Water-based polyurethane | 110-150 | Thinner solids, more coats to the same build |
| Wiping varnish, thinned blends | 130-190 | Very thin coats by design, many of them |
| Penetrating oil, Danish or tung | 100-160 | Wildly variable, most of the first coat goes into the wood |
| Hardwax oil | 200-350 | Applied extremely thin and buffed back; over-application is the usual mistake |
| Shellac, 2 lb cut | 140-200 | Fast flash-off, thin build per coat |
| Brushing lacquer | 80-120 | High solvent content, low solids |
| Oil-based stain, wiped | 100-150 | Depends entirely on how much you wipe off |
| Gel stain | 70-110 | Heavy bodied, sits on the surface |
| Semi-transparent deck stain | 50-100 | Rough or weathered wood at the bottom of the range |
Application method shifts these. Wiping a coat on and immediately off leaves less material behind than a brush does, so a quart goes further. A spray gun is the opposite: HVLP transfer efficiency on furniture parts is realistically around 60 to 80 percent, and an aerosol can is worse, with a good fraction of every pull of the trigger going past the work as overspray. Those factors are applied to the coverage figure rather than to the area.
Working the area out properly
The commonest way to underbuy is to count the top of the table and forget everything else. A 60 by 30 inch tabletop is 12.5 square feet on the show face. Finish the underside as well, which you should, and it is 25. Add the four edges at three-quarters of an inch thick and it is 25.9. Then there are four legs and four aprons, and the job is nearly double what you first thought.
Finish the underside of any solid top even though nobody sees it. It is not about appearance. A top sealed on one face only exchanges moisture faster through the bare face, the two faces end up at different moisture contents, and the top cups. The finish is doing a mechanical job there, not a cosmetic one.
Buying it
Buy for the top of the range and buy it in one purchase. Two things go wrong otherwise. Stains and pigmented finishes vary between batches, sometimes visibly, so a second can bought later is a gamble on a piece that is already half finished. And an oil-based finish that has been opened and half used starts skinning and thickening, so the leftovers from the last job are rarely as good as they look.
The one exception is oil-based finishes on small jobs, where opening a fresh quart to use four ounces guarantees the remaining 28 ounces will gel in the can before you next need them. For work under about eight square feet, half-pints and pints exist for a reason.
Whatever the number here says, it is an estimate with a band around it that no amount of arithmetic will narrow. Wood porosity is not a published constant, and the difference between a heavy hand and a light one on a brush is larger than the difference between two brands of polyurethane. Use the figure to decide between a pint and a quart, not to decide whether you have enough left in the can.
Questions people ask
How many coats of polyurethane does a tabletop need?
Three is the usual answer for oil-based poly on a table that will see normal use, and four or five on a kitchen table or a desk that gets worked on. Water-based products build thinner, so add a coat or two to reach the same film thickness. What matters more than the count is what happens between coats: scuff-sanding with 320 or 400 grit knocks down dust nibs and gives the next coat something to grip, and skipping it on a fully cured previous coat is how you get a finish that peels in sheets later. If a fifth coat is going on because the third and fourth looked uneven, the problem is application, not quantity.
Why did my stain use twice what the can said?
Most likely bare open-grained or blotchy wood, plus a wipe-on-wipe-off technique that leaves more in the can than on the wood. The published figure assumes an even, sealed surface and a thin application. Oak drinks through its open pores, pine and cherry take stain unevenly and tempt you into flooding the surface to fix it, and end grain will absorb until you stop feeding it. A pre-stain conditioner or a thin wash coat of dewaxed shellac cuts absorption considerably and, on blotch-prone species, is the difference between an even colour and a mottled one.
Does spraying use more finish than brushing?
Yes, in total material, though it puts a thinner and more even film on the work. The loss is transfer efficiency: some fraction of what leaves the gun never lands on the part. A well-tuned HVLP gun on flat panels might get 70 to 80 percent onto the work, a conversion gun on small spindly parts considerably less, and an aerosol can lower still. Against that, spraying gives a better film with fewer coats and no brush marks, so the finished job may take a similar number of coats and simply waste more per coat. If you are spraying, buy generously.
Can I just buy a gallon and be safe?
For a large job, yes, and it is usually cheaper per ounce. For a small one it is a false economy, because opened oil-based finishes have a short shelf life once air gets to them. A gallon of polyurethane used a quarter at a time will be a skinned, gel-flecked mess long before the last quarter comes out. Water-based finishes keep somewhat better but still degrade. Shellac is the strictest of all: mixed shellac has a limited pot life and old shellac will not cure properly, so mix it from flakes or buy small and check the date on the can.