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What coating solids means
“Solids” are the nonvolatile ingredients that remain in the cured coating film after volatile material leaves during application and curing. Depending on the product and method, the value may be measured by a laboratory test or calculated from formulation data. ASTM D2832-25 is a guide to determining volatile and nonvolatile content for paint and related coatings, including low-VOC and multicomponent materials.
A solids percentage must always state its basis:
- Weight solids is the mass of nonvolatile material divided by the mass of the coating basis being reported.
- Volume solids is the volume of cured, nonvolatile coating divided by the volume of the coating basis being reported.
Weight percent is not a synonym for volume percent. Ingredients have different densities, so two coatings can have similar weight-solids values but different film-forming volume. ASTM D5201-26 covers formulation calculations for weight and volume solids, density, solvent content, VOC and hazardous-air-pollutant content, including multicomponent coatings.
As supplied versus as applied
“As supplied” refers to the material in the container, before a specified reduction or mixing step. “As applied” refers to the mixture actually put on the surface, such as a two-component product after its components are combined or a coating after permitted thinner is added. A solids value is incomplete unless the basis and preparation state are identified.
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- 3-4 times the coverage of standard aerosol spray paint
- Does not run or sag
- Protects against rust, weather, oil, gasoline and most corrosive chemicals
- Resists abrasion and heat
- Has single coat hiding
Are high-, medium- and low-solids categories universal?
No. The labels are useful shorthand, but their numerical boundaries depend on a named regulation, standard, market segment or product specification. Historical U.S. EPA technical guidance illustrates why a single definition is unsafe: it described a Los Angeles County Rule 66 exemption using 80% solids by volume and also discussed an industry-proposed 70% by volume threshold. Those 1976 figures are historical examples, not a current worldwide definition of “high solids.”
The same historical guidance noted application difficulties as viscosity increased. A formulation can therefore have a high solids percentage yet require equipment, temperature control, thinning limits or application techniques that differ from a lower-solids product. Always use the category definition attached to the specification or regulation governing the job.
| Label | What it tells you | What it does not establish |
|---|---|---|
| Low solids | A relatively larger share of the reported coating basis is volatile material under the stated method. | It does not by itself establish a universal percentage, VOC compliance, durability or ease of application. |
| Medium solids | An intermediate product classification used by a particular manufacturer, market or specification. | There is no single numerical band that applies everywhere. |
| High solids | A formulation designed to deliver more nonvolatile film from the stated coating basis, often with less solvent than conventional products. | It does not guarantee a particular VOC value, application method, cure speed or performance. |
How to compare VOC when solids differ
VOC figures are comparable only when their denominator and calculation method match. ASTM D3960-25 describes VOC on several bases, including volume of coating (with water and exempt volatile compounds treated as specified), volume of coating solids and mass of coating solids.
Rank #2
- 3-4 times the coverage of standard aerosol spray paint
- Does not run or sag
- Protects against rust, weather, oil, gasoline and most corrosive chemicals
- Resists abrasion and heat
- Has single coat hiding
For coatings used for the same application at the same application efficiency, ASTM D3960-25 states that VOC expressed per volume of coating solids gives a linear measure of differences in VOC released. Its example compares 3 lb of VOC per gallon of solids with 6 lb per gallon of solids: under that assumption, the first releases half as much VOC.
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A VOC value reported per gallon of coating, excluding water and exempt compounds, does not account for different volume-solids levels. Before comparing products, record:
- the VOC denominator (coating, volume of solids or mass of solids);
- how water and exempt volatile compounds are treated;
- whether the figure is measured or calculated and which method was used;
- the application efficiency assumed; and
- the jurisdiction, regulation and date that control compliance.
Formulation calculations are not automatically regulatory determinations. ASTM D5201-26 cautions that calculated values may or may not be accepted by a particular regulation. Some rules use special calculations; for example, Canada’s architectural-coatings regulation provides a distinct calculation for low-solids coatings. A product’s technical data sheet and the applicable authority should be checked together.
Rank #3
- 3-4 times the coverage of standard aerosol spray paint
- Does not run or sag
- Protects against rust, weather, oil, gasoline and most corrosive chemicals
- Resists abrasion and heat
- Has single coat hiding
What “high solids” changes in practice
Film build and coverage
Because more of the applied material can remain as film, a higher volume-solids product may achieve a specified dry-film thickness with less wet material than a lower-solids product. The actual result depends on the product’s stated solids basis, application loss and the thickness specification.
Viscosity and application
Reducing volatile content can increase viscosity. The historical EPA guidance recorded reported application problems as solids and viscosity increased. Confirm allowable reduction, spray-tip or pump requirements, pot life, temperature limits and curing conditions with the product specification rather than assuming that all high-solids coatings apply alike.
Emissions and compliance
High-solids is not the same as “zero VOC.” Waterborne, UV-cured, high-solids and powder coatings are examples of approaches EPA identifies as low- or no-VOC/HAP options, but their chemistry, curing, performance and regulatory treatment are not identical. Compliance still depends on the applicable rule and calculation method.
Rank #4
- INDUSTRIAL METAL PRIMER: Designed for steel and metal surfaces to help improve topcoat adhesion and provide a protective base coat.
- RUST INHIBITIVE COATING: Helps protect metal surfaces from rust and corrosion when used as part of a coating system.
- FAST DRYING FORMULA: Dries quickly to help reduce downtime and allow faster recoating and project completion.
- HIGH BUILD PRIMER: Provides strong coverage and helps fill minor surface imperfections for a smoother topcoat finish.
- COMPATIBLE WITH METAL SURFACES: For use on steel, iron, machinery, equipment, pipes, structural metal, and other properly prepared metal surfaces.
What is a conversion coating?
A conversion coating is a chemical or electrochemical treatment that reacts with a metallic surface and forms a surface layer containing a compound of the metal. It is a substrate treatment, not a solids category for a liquid paint.
EPA describes conversion treatments as serving several purposes:
- corrosion protection or retardation;
- improved lubricity;
- preparation for subsequent coatings; and
- a specified appearance.
Phosphate conversion coatings
Phosphating can form a phosphate layer on steel or zinc-plated steel. That layer can provide a base for paints and other organic coatings, helping the later film adhere to a prepared surface. The phosphate layer itself should not be described as the paint’s weight-solids or volume-solids percentage.
Best Value
- 4X THE COVERAGE: Covering 18 SQ. FT., this high-solids formula delivers 4 times the coverage of standard aerosol spray paint, reducing cans needed per job
- NO-DRIP FORMULA: Does not run or sag during application for a smooth, professional finish
- RUST & CHEMICAL PROTECTION: Protects against rust, weather, oil, gasoline, and most corrosive chemicals
- SINGLE-COAT HIDING: Full opacity and coverage in just one coat, saving time on application
- HEAT RESISTANT COATING: Withstands temperatures up to 300-Degree F while resisting abrasion and high humidity
Chromate conversion coatings
Chromate treatments use chromium compounds on substrates including aluminum, zinc, cadmium and magnesium. ASTM B921-08(2018) covers rinsed and non-rinsed non-hexavalent chromium conversion coatings on aluminum and aluminum alloys for corrosion retardation and as a base for organic films such as paints, plastics and adhesives. Hexavalent and non-hexavalent processes are distinct; selection and safety requirements must follow the current process specification and applicable regulations.
How the two concepts fit together on a project
- Identify the substrate. Determine whether the surface is steel, galvanized steel, aluminum or another metal and whether a conversion treatment is specified.
- Prepare and treat the metal. Clean and condition the surface, then apply the specified conversion coating when required. Follow the process controls for concentration, temperature, contact time, rinsing and drying.
- Select the organic coating. Read its technical data for weight or volume solids, as-supplied or as-applied basis, VOC denominator, permitted reduction and cure requirements.
- Verify the governing rule. Match the product’s test or calculation method to the jurisdiction and project specification; do not infer compliance from a solids label alone.
- Apply and inspect. Control wet-film and dry-film thickness, environmental conditions and application efficiency. A conversion layer supports the substrate; the paint or other organic coating supplies the subsequent film.
A practical comparison checklist
- Is the solids figure by weight or by volume?
- Is it measured or calculated?
- Is the material as supplied, mixed, or as applied after reduction?
- For a multicomponent product, are all components included?
- What denominator is used for the VOC number?
- Which regulation, standard, edition and jurisdiction govern the job?
- Does the specified solids level create viscosity, equipment or cure constraints?
- Does the metal require a phosphate, chromate or another conversion treatment before the organic coating?
- Are the conversion process and the paint specification compatible, including rinsing, drying and recoat timing?
The key distinction
Solids content answers, “How much of this coating becomes the film?” Conversion coating answers, “What chemical surface layer is formed on the metal before the next coating?” A sound specification keeps those questions separate while coordinating them: it names the solids basis and calculation method for the organic coating, and it separately identifies the metal pretreatment and its process requirements.
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