Film Thickness and Why Mils Matter
A Mil Is One Thousandth of an Inch
Film thickness in the coatings trade is measured in mils. One mil is 0.001 inch, roughly a quarter of the thickness of a sheet of household aluminium foil. It sounds trivially small until you realise that the entire difference between a floor that survives a decade of use and a floor that wears through in two years is often six or eight mils of material.
Thickness is the physical quantity of resin standing between traffic and the concrete, and it is the most useful figure to ask about when someone in Allen quotes you a garage floor.
Typical Builds
Residential garage systems commonly land somewhere in the range of 8 to 15 mils of total dry film across primer, body coat and topcoat. A flake broadcast system runs thicker, because the flake itself adds height and the clear coats have to bury it, and those often reach 20 to 30 mils.
Light commercial and workshop specifications frequently start where residential ends, in the 20 to 40 mil range. High build systems, meaning troweled mortars and self levelling resins, are measured in fractions of an inch rather than mils, typically 1/8 to 1/4 inch, and need different equipment and a different crew skill set.
Coverage Rate Is the Thing That Sets Thickness
Thickness is not set by intention, it is set by arithmetic. A gallon of coating contains a fixed volume of solid material. Spread that gallon across a smaller area and the film is thicker. Spread it across a larger area and the film is thinner. For a coating that is close to 100 percent solids, one gallon spread over 160 square feet gives roughly 10 mils, and the same gallon spread over 320 square feet gives roughly 5 mils.
That is the whole mechanism. Coverage rate in square feet per gallon and film thickness in mils are two ways of stating the same fact.
How a Careful Crew Checks
Wet film thickness gauges are cheap combs, usually a small notched metal or plastic card. The installer presses the comb into the wet coating and reads which teeth came back wetted, which gives the wet film thickness on the spot. Dry film is then calculated from the solids content of the product.
Timing is the reason this matters. Once the resin has cured, thickness can only be measured destructively. A crew that checks as it goes can correct a thin pass while correcting it is still free.
The Quiet Way a Floor Gets Shortchanged
Stretching material is the most common shortcut in this trade, and it is popular precisely because it is invisible. A floor spread at 6 mils and a floor spread at 12 mils look identical on the day of handover. Same colour, same gloss, same smell in the garage. The difference only surfaces when the thin one starts showing tyre marks and hot tyre pickup where the thick one does not.
This is why the quantity of material used on a job is a more honest specification than any adjective. Square footage divided by gallons applied is a number that cannot be argued with, and it is worth knowing for any floor in Collin County or anywhere else.
What Extra Thickness Actually Buys
More film gives you more abrasion life, because abrasion consumes material and there is more of it to consume. It gives better impact resistance, because a thicker film distributes the energy of a dropped object across a wider area of the slab instead of concentrating it. It gives better resistance to chemical attack, because a spill has more layers to work through before it reaches concrete.
- Abrasion: a wear surface lasts in proportion to how much of it there is.
- Impact: thicker films spread point loads rather than cracking at them.
- Chemistry: depth buys time to clean a spill before it does damage.
- Forgiveness: minor slab imperfections disappear under a higher build.
None of that means thicker is always correct. A quiet residential garage does not need a troweled mortar system. The point is to know what build you are getting, as a number rather than as a description.