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Coating solids describe how much of a liquid coating remains as nonvolatile film-forming material; a conversion coating is a separate chemical or electrochemical treatment that changes a metal surface before a later coating is applied. To compare products meaningfully, check whether a solids figure is by weight or volume, whether it describes material as supplied or as applied, and how any VOC figure is calculated. “High,” “medium” and “low” solids are not universal numerical categories.
What does coating solids mean?
Solids are the nonvolatile materials in a liquid coating that remain to form the film as volatile components leave during application and curing. Depending on the method, nonvolatile content may be measured or calculated from formulation data. ASTM D2832-25 is a guide to methods for determining volatile and nonvolatile content in paints and related coatings, including low-VOC and multicomponent coatings: ASTM D2832-25.
A solids percentage is meaningful only when its basis is clear. Weight solids and volume solids are different quantities, not alternate names for the same percentage. ASTM D5201-26 covers formulation calculations for both, along with other physical constants, and addresses values for materials as supplied or as applied, including multicomponent coatings: ASTM D5201-26.
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- Solids by weight compares the mass of nonvolatile material with the mass of the coating on the stated basis.
- Solids by volume compares the volume of the resulting nonvolatile film-forming material with the coating volume on the stated basis.
- As supplied versus as applied matters when a product is mixed with another component or thinned before use. The reported basis should make clear which material condition is being described.
Are low-, medium- and high-solids coatings defined by fixed percentages?
No single numerical boundary applies everywhere. The labels are relative unless a named regulation, standard, market convention or product specification defines them. The application method also matters: increasing solids can mean higher viscosity, which may affect whether a coating can be applied by conventional means.
Historical U.S. EPA guidance from 1976 gives examples of how definitions have differed: it describes an 80% solids-by-volume threshold used for an exemption under Los Angeles County Air Pollution Control District Rule 66, and an industry proposal for a 70% solids-by-volume threshold. These are historical, attributed examples—not current universal definitions of high solids. The guidance also records concerns about application as viscosity increased: U.S. EPA, Control of Volatile Organic Emissions from Existing Stationary Sources, Volume 1 (1976).
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For that reason, a product described as “medium solids” cannot be assigned a reliable percentage from the label alone. Look for the technical data sheet’s stated solids basis and value, then check the application instructions and any applicable local definition.
How should you compare VOC figures across coatings?
First check the denominator: a VOC value per volume of coating is not directly equivalent to one per volume or mass of coating solids. Also check whether water and exempt volatile compounds are excluded, what test or calculation method was used, and which jurisdiction’s rules apply.
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ASTM D3960-25 explains that expressing VOC per volume of coating solids gives a linear comparison of VOC released by different coatings used for the same application, assuming the same application efficiency. Its example compares 3 lb VOC per gallon of coating solids with 6 lb per gallon of coating solids: under that assumption, the first coating would release half as much VOC. A figure expressed per volume of coating, excluding water and exempt volatile compounds, does not account for differences in volume solids. See ASTM D3960-25.
That comparison is not a universal emissions prediction: different application efficiencies or uses can change actual releases. Nor does a formulation calculation automatically establish regulatory compliance. ASTM D5201-26 notes that calculated values may or may not be acceptable under a particular regulation, so verify the method and jurisdiction that govern the job.
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Regulatory methods can treat particular product categories differently. For example, Canada’s architectural coatings regulation includes a distinct calculation for low-solids coatings; that is a jurisdiction-specific rule, not a worldwide convention: Government of Canada, Volatile Organic Compound (VOC) Concentration Limits for Architectural Coatings Regulations, section 12.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is a conversion coating, and what does it do before painting?
A conversion coating forms when a metallic surface is treated chemically or electrochemically, producing a surface layer that contains a compound of the metal. It is a treatment of the substrate, not a category of liquid paint and not a solids percentage.
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Depending on the process and substrate, a conversion treatment may provide corrosion protection, lubricity, a special appearance or preparation for an additional coating. The U.S. EPA describes chemical etching as depositing a conversion coat to improve corrosion and adhesion properties before paint is applied: U.S. EPA, Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency (2021).
Phosphate conversion coatings
Phosphating can form a phosphate layer on steel or zinc-plated steel. In industrial pretreatment, that layer can serve as a base for paint and other organic coatings. It is a surface-preparation step beneath the later film, rather than part of the liquid paint’s solids calculation. The EPA discusses phosphating and industrial pretreatment in its industrial assessments guide.
Chromate conversion coatings
Chromate conversion treatments are another family and involve chromium compounds; EPA describes their use on metals including aluminum, zinc, cadmium and magnesium. Do not treat all chromate processes as interchangeable. ASTM B921-08(2018), for example, covers rinsed and non-rinsed non-hexavalent chromium conversion coatings on aluminum and aluminum alloys, including use for corrosion retardation and as a base for organic films such as paints, plastics and adhesives: ASTM B921-08(2018). This scope does not by itself establish a complete current safety or regulatory comparison with hexavalent processes.
How to read a coating specification
When evaluating a coating for a project, use the product data and the requirements that actually apply rather than relying on a broad solids label.
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- Find the solids value and basis. Confirm whether it is weight or volume solids.
- Check the material condition. Determine whether the value is for the product as supplied or after mixing, thinning or other preparation.
- Read VOC with its denominator. Note whether the figure is per coating volume, coating-solids volume or coating-solids mass, and how water and exempt compounds are treated.
- Confirm the method and jurisdiction. A formulation calculation and a regulatory compliance determination are not automatically the same thing.
- Check application constraints. Review viscosity, application method and curing requirements; a solids percentage alone does not establish that a coating suits a particular process.
- Assess the substrate separately. For metal, determine whether a conversion treatment is specified or needed beneath the organic coating.
Waterborne, UV-cured, high-solids and powder coatings are among the approaches the EPA identifies for low- or no-VOC/HAP coatings. They are distinct coating families, not interchangeable formulations with identical chemistry, curing, performance or regulatory treatment. In the EPA’s covered context, VOC may be determined from formulation data or EPA Method 24: EPA, Monitoring by Control Technique—Compliant (Low/No VOC/HAP) Inks and Coatings.
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