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 sets coverage directly, which is why comparing two products by price per gallon says little until their solids are compared. Here is the same 12.0-mil target at volume solids from 30 to 100 percent.

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 the spread rates on this page are built from. So when a product's published coverage does not reconcile with the arithmetic above, check which film thickness and which solids figure that coverage assumes before blaming the product.

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 a kit label can quote the mixed volume you will end up with or the resin side only, with the hardener 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

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 multiplying it by the volume solids fraction gives the dry film 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.

It is also a cheap check against spreading the material too far because the bucket looked as though it would not stretch.

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

The federal architectural coatings table has a floor-coatings row at 400 grams per litre and an industrial-maintenance row at 450, a difference of 50 grams per litre. Which row applies follows the category a product is formulated and recommended for. The regulation's floor-coating definition names flooring, including decks, porches and steps in a residential setting. Its industrial-maintenance definition covers high-performance coatings for surfaces exposed to extreme conditions in an industrial, commercial or institutional setting, and section 59.405(b) requires those products to carry a label description such as industrial use only or not for residential use. The regulation has no certification step; the label states the VOC content or the limit the product complies with.

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 limit 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 recommends. A waterborne product is therefore not inside the limit merely because the tin holds a lot of water, and thinning a product past its data sheet takes it off the basis the limit is measured on.

Two things follow for a domestic slab. Comparing VOC figures across products only means anything when both are quoted on that basis; a figure quoted as supplied is a different measurement in the same units. And the table limits what may be sold; it is not a measure of what is comfortable to breathe in a closed garage, so ventilate as the product's data sheet directs.

Section 59.410 of the same regulation leaves states and local governments free to adopt their own limits, so check the rule where you live as well as the federal row.