Wet and dry film thickness: calculation, measurement and tolerances

Dry film thickness equals wet film thickness multiplied by the product's volume solids, corrected for thinning when the product is thinned. The applicator therefore checks the wet film during application with a notch gauge, and the inspector verifies the dry film after drying with a magnetic or electronic gauge, per SSPC-PA 2 or the standard named in the specification. The target ranges always come from the specification and the data sheet.

Thickness is the most measured property of an industrial coating, and yet one of the least understood. A film that is too thin lets moisture through; a film that is too thick cracks, traps solvent or disbonds. This guide explains, for maintenance managers, project managers and inspectors in Quebec and Ontario, how wet and dry film relate, how to measure and how to respond to a deviation. It complements our coating inspection page and our reference page on protective coatings.

What is the difference between wet film and dry film?

Wet film is the coating thickness right after application, solvents included; dry film is what remains once the solvents have evaporated and the product has cured. Only the dry film protects the steel, and it is the dry film that the specification requires.

Wet film is measured immediately, which lets you correct application while it is happening. Dry film is measured after drying, when it is too late to correct without adding a coat or sanding. The two measurements are therefore complementary: one guides the painter, the other verifies the result. The dry film thickness glossary entry summarizes the concept.

How do you calculate dry film from wet film?

Multiply the wet film by the volume solids, expressed as a fraction. If the product is thinned, also divide by one plus the thinning ratio by volume. The inverse relationship gives the wet film to target for a given dry film.

Three precautions prevent calculation errors:

• Volume solids, not weight solids. The data sheet often gives both; only volume solids applies to thickness.
• Actual thinning. Thinner added on site reduces the solids of the mix. It is recorded for every batch, within the limit allowed by the data sheet.
• Solvent-free products. With very high volume solids, wet and dry film are close, but the calculation still has to be done.

The relationship is theoretical: film porosity, substrate roughness and application losses explain small differences. That is why dry film is always measured.

Which gauges are used for wet and dry film?

Wet film is measured with a notch gauge per ASTM D4414; dry film on steel is measured with a non-destructive magnetic or electronic gauge per ASTM D7091 and SSPC-PA 2, or ISO 19840 and ISO 2808, depending on the specification.

• Wet film notch gauge. Pressed squarely into the fresh film; the last wetted tooth gives the thickness. The reading is taken at once, before the solvents evaporate.
• Magnetic pull-off gauge. A mechanical instrument that measures the force needed to lift a magnet off the film.
• Electronic gauge. Magnetic or eddy current probe, with reading storage and statistics. This is the standard instrument on job sites.
• Destructive measurement. A cut examined with a graduated magnifier lets you measure each coat, when the specification calls for it or a dispute requires it.

On blast-cleaned steel, the surface profile affects the reading: SSPC-PA 2 provides for a base metal reading on bare blasted steel, or a gauge adjustment, to correct for this effect.

How is dry film measured per SSPC-PA 2?

SSPC-PA 2 sets out how to verify the gauge, how many readings to take, how to group them and how to judge conformance. The principle: several readings form a spot measurement, several spot measurements characterize an area, and conformance is judged on the measurements, not on a single reading.

• Gauge verification. Before and after each session, on standards or shims of known thickness, following the gauge manufacturer's instructions.
• Profile correction. A base metal reading on the blasted steel, or an adjustment on shims placed on that same steel.
• Readings and spot measurements. The number of readings per spot and spots per area follows the standard's sampling plan.
• Restriction levels. The standard offers several tolerance levels around the minimum and maximum thickness; the specification selects the one that applies.
• Complex shapes. Edges, bolts and small parts have specific provisions.

Readings are recorded by coat and by area. Our article on coating inspection links this measurement to adhesion and holiday testing.

What happens when the film is too thick or too thin?

A film that is too thin protects poorly because it leaves pores and poorly covered profile peaks; a film that is too thick builds internal stress, traps solvent and can crack or disbond. Both deviations shorten the service life of the system.

Inorganic zinc primer is especially sensitive to excess thickness, as our inorganic vs organic zinc primer comparison explains. Our guide to coating failure modes helps link a visible defect to a thickness cause. In immersion service, a thin film is also detected by the holiday test.

How do you prevent thickness deviations on site?

Prevent them by calculating the target wet film before starting, measuring it continuously during application and checking the dry film coat by coat, before the next one. Inspection only at the end of the job finds deviations too late.

• Before application. Calculate the target wet film from the volume solids and the planned thinning; set up the spray equipment.
• During application. Frequent notch gauge readings, especially on edges, in corners and on welds, where film varies most.
• Stripe coating. Edges and welds receive a brush coat before the full coat to avoid thin areas.
• After each coat. Measure and record dry film before releasing the next coat.

The systems described in our guide to ISO 12944 paint systems list a nominal thickness per coat; that is the figure the wet film calculation should target.

Frequently asked questions

Can you rely on wet film measurement alone?

No. It guides application, but the required thickness is the dry film, which also depends on the actual volume solids, thinning and the substrate. Dry film is always measured after drying.

Why do two gauges give different readings?

Because their verification, profile correction or probe differ. Verify each gauge on standards, apply the same profile correction and follow the same sampling method.

Does a single reading below the minimum make the area nonconforming?

Not necessarily. Under SSPC-PA 2, conformance is judged on spot measurements and on the area, using the restriction level chosen in the specification. A single low reading does, however, call for additional measurements.

How is dry film measured on concrete?

Magnetic gauges do not work on concrete. A suitable ultrasonic gauge, a destructive measurement or a check of product consumption per area is used, depending on the specification.

How do you get a quote for an inspection or coating work?

Our crews and qualified subcontractors measure thickness, keep inspection records and apply coating systems as part of our industrial painting services, including in Quebec City, Montreal and Ontario. For a thickness dispute, our article on industrial coating warranties shows which records to keep. Send the specification, surface areas and schedule through the request a quote form: pricing on consultation, reply within 24 business hours.