Wet film, dry film, and how far a gallon really goes

What volume solids does to your material bill

Volume solids is the fraction of the wet product that stays behind as film once the carrier has gone. It decides coverage completely, and it is why comparing two products by price per gallon is meaningless. Here is the same 12.0-mil target across the range of products actually on sale.

At one hundred percent solids the wet and dry numbers are identical, which is the only case where a wet-film gauge reads the finished thickness directly. Below that, the gauge reads high and you subtract nothing — you simply lay more on and let the difference leave.

Everything in that table stands on one definition, and it is a legal one rather than a coatings one. A gallon is a fixed volume, so the area a gallon can cover at a given thickness is arithmetic and not a claim.

Statements of fluid measure shall be in terms of both the U.S. gallon of 231 cubic inches and quart, pint, and fluid ounce subdivisions thereof and SI metric liters or milliliters

What that changes for your floor Divide that volume by a thickness of one mil and a gallon of pure solids covers 1,604 square feet — the constant every spread rate on this page is built from. So when a product's published coverage does not reconcile with the arithmetic above, the gap is almost never the product: it is a thinner film than the one you intend to lay, stated somewhere in smaller type on the same data sheet.

What that definition does not settle is how much material is in the box. A gallon is a unit of measure, not a description of a two-part kit, and kits are labelled both ways: some quote the mixed volume you will end up with, others quote the resin side only and the hardener arrives on top of it. The takeoff wants the mixed figure. Entering the label where the label means the resin side alone under-orders the job by the whole hardener fraction, which on a hundred-percent-solids product is not a rounding error.

Using a wet-film gauge, which costs almost nothing and settles arguments

A wet-film gauge is a small notched comb. Press it into the wet coating and read the highest tooth that comes out wetted. That reading is the wet film, and dividing it by the volume solids fraction is how you know what you will be left with — while there is still time to do something about it.

1. Mix a batch and pull it out at your intended rate.
2. Take a gauge reading in three places across the batch, not one.
3. Compare against 26.7 mils wet. Reading low means you are stretching the material; reading high means you will run out.
4. Adjust the spread and re-check on the next batch, not at the end of the coat.

This is also the only cheap defence against the most common quiet failure on a domestic job, which is not a bad product or a bad slab but simply spreading the material too far because the bucket looked like it would not stretch.

The VOC rows, and why the category on the label matters

Two-part resinous floor products are sold under two different rows of the federal architectural coatings table, and the rows carry different ceilings. A product certified as a floor coating must meet 400 grams per litre; one sold as an industrial maintenance coating may go to 450, a difference of 50 grams per litre.

Both of those figures are quoted from the same table, and the table carries a headnote that decides what the numbers are measured on. It is worth reading before comparing two products by their VOC figures.

Unless otherwise specified, limits are expressed in grams of VOC per liter of coating thinned to the manufacturer's maximum recommendation excluding the volume of any water, exempt compounds, or colorant added to tint bases.

What that changes for your floor The ceiling is not measured on the pail as you buy it: water and exempt compounds come out of the denominator first, and the coating is measured thinned as far as the manufacturer will sanction. A waterborne product is therefore not compliant merely because most of what is in the tin is water, and thinning a product past its data sheet to make it spread on a hot afternoon takes it off the basis on which it was certified.

Two things follow for a domestic slab. Comparing VOC figures across products only means anything when both are quoted on that basis; a marketing figure quoted as-supplied is a different measurement wearing the same units. And the table sets a ceiling on what may legally be sold, not a floor on what is comfortable to breathe in a shut garage — a product well inside the limit can still make a closed space genuinely unpleasant, and occasionally dangerous, for a few hours.

Several states and air districts set stricter limits than the federal ceiling, so a product legal to sell nationally may not be legal to sell where you live.