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Solids Content in Epoxy and Why Film Thickness Varies
The number on the label that explains the floor
Among the small print on an epoxy data sheet sits a line that reads solids by volume. It is easy to skip, and it is one of the most useful numbers in the whole document. It tells how much of the liquid in the bucket stays on the floor after the coating cures. The rest leaves as vapor. Knowing the figure turns a vague idea of thick or thin into something that can actually be calculated.
What solids means
Every liquid coating is made of two kinds of material. Solids are the parts that remain: resin, hardener, pigment and fillers. Volatiles are the parts that evaporate: water, solvents and thinners. A coating that is mostly solids leaves a film nearly as thick as the wet coat spread on the floor. A coating that is mostly volatiles shrinks dramatically as it dries.
The measure is usually given by volume, because thickness is a volume matter. A product with half its volume as solids will leave a film roughly half the thickness of the wet layer. A product with nearly all its volume as solids, sometimes called a one hundred solids epoxy, leaves a film essentially as thick as the wet coat.
From wet film to dry film
Installers talk about wet film thickness and dry film thickness. Wet film is what is applied, measured with a notched comb gauge pressed into the fresh coat. Dry film is what remains after cure. The relationship is simple: dry film equals wet film multiplied by the solids fraction. A wet coat of ten thousandths of an inch of a product with half solids leaves five thousandths dry.
This is why spread rate is such an important concept. A manufacturer will state how much area a gallon covers at a given dry film thickness. Spreading the product more thinly to stretch a bucket reduces the film below what the product was designed to give, and the floor suffers for it.
Why high solids is not always better
A full solids epoxy gives thickness in one pass, but it has drawbacks. It is more viscous, less forgiving, and its short working time makes large areas harder to manage. It can be less penetrating as a primer, since thick liquid does not flow into pores as easily. Lower solids products flow and level easily and may be better where a thin, penetrating layer is wanted.
So the right solids content depends on the layer. A primer is often lower in solids because the job is to soak in. A body coat is higher in solids because the job is to build film. A clear wear layer lies somewhere in between, balancing flow and thickness.
Where thickness variation comes from
Even with a good product, film thickness on a real floor varies for several reasons.
- Slab texture. A rough, open surface absorbs more liquid and leaves a thinner film on the high points. A smooth surface holds more.
- Spread technique. Pressing hard on a squeegee leaves less behind than a light pass. Back-rolling pushes and redistributes.
- Slope and low spots. Liquid flows downhill, so a pitched floor has thicker film at the low end.
- Temperature. Warm liquid flows more, thinning at peaks; cold liquid stays put.
- Edges and obstacles. Cut-in work with a brush tends to be thicker than rolled areas.
Some variation is normal and invisible. Large variation shows up as shiny patches, dull patches and weak spots that wear first.
Measuring the result
Dry film thickness can be measured with a gauge, but on concrete the options are limited because the usual magnetic tools work on metal. A more practical method is to check wet film during application with a comb, and to track how much material is used compared with the area covered. If a bucket claimed to cover a certain area runs out well before reaching it, the film is thicker than expected. If material remains after the area is done, the film is thinner.
Reading a data sheet for thickness
Look for three numbers: solids by volume, recommended dry film thickness per coat, and theoretical coverage. Coverage figures are theoretical because they assume a perfectly flat, non-porous surface and no waste. Real coverage on concrete is lower, often noticeably so, since the slab soaks in some product and some stays in the bucket and on tools. Good planners add a margin for this.
A product that gives no solids figure at all is a warning sign. It may be hiding a low number. A product that is open about it is easier to judge and compare.
Thickness and durability
Thicker films bring practical benefits. They bridge small slab imperfections. They resist wear by providing more material to lose. They distribute pressure from heavy loads. They also run a little more risk of trapped bubbles and heat buildup during cure, which is why very thick layers are often split into separate coats. There is a balance, and the data sheet range is the manufacturer's recommendation for it.
Too thin is the more common problem. A film that looks full when wet may be sparse when cured. The difference can be a factor of two or more for a low-solids product. Owners who compare estimates should ask how thick the film will be after cure, not just how many coats will be applied.
A worked example in plain words
Imagine two buckets of the same size and the same color. The first holds a product with about half of its volume as solids. The second holds a product that is almost entirely solids. Spread over the same floor at the same wet thickness, the first leaves a film about half as thick as the second once the vapors have left. The floors look alike on the day of application, shiny and even, but the thinner one has half the material to lose before the concrete shows. Years later, the difference appears around the wheel paths and the door, where wear is greatest. This is the practical meaning of the solids figure, and it is why a careful buyer asks for it before comparing bids or products by the bucket.
Putting it together
Solids content is a plain idea with large consequences. It separates a coating that leaves a durable film from one that mostly disappears into the air. It also explains why two floors with the same color and the same number of coats can feel and last so differently. A short look at the number on the data sheet, and a question or two about spread rate, reveals more about a coating than any amount of marketing language about shine or strength.