# Wet painting

Specifying liquid-applied industrial paint systems through preparation, application, mixing, cure, film build and acceptance.

Reviewed: 2026-11-01

Tags: processes, finishing, liquid-coatings, painting

A hypothetical **120 µm dry-film** target at **60% volume solids** needs **200 µm wet**. Adding permitted solids-free thinner at **10% of original mixed paint volume** lowers solids to **54.545%**, so the target needs about **220 µm wet**. Applying 200 µm of the diluted mix gives about **109.1 µm dry**. Inspect the cured dry film. [Hempel calculation guidance, page 3](https://www.hempel.com/-/media/Hempel/Files/Technical-Guidelines/Application-Methods/Mixing--thinning--2026.pdf)

## What the process specifies

The WBDG identifies the binder as the film-forming component. Specify industrial paint by its binder, preparation, application and cure. Clear systems can be pigment-free and formulations can be solvent-free. [WBDG paints and coatings knowledge area](https://www.wbdg.org/dod/cpc-source/paint-coatings-knowledge-area)

Specify the **complete coating system**: substrate and preparation, primer, intermediate/topcoat where required, each product and thickness, application route, cure and service conditions. A colour code or resin-family name does not establish compatibility, corrosion performance, chemical resistance or a repair procedure.

For structural-steel corrosion protection, **ISO 12944-1:2017**, edition 2, November 2017, is the general introduction to the ISO 12944 series. Its record reports confirmation in 2023 and a revision project; a project is not a published replacement. The standard's steel-structure scope should not be silently extended into a universal finish specification for plastics, electronics or precision mechanical parts. [ISO 12944-1 public scope](https://www.iso.org/standard/64833.html)

## Select the coating and application together

Distinguish the product's carrier, binder and cure requirements. “Water-borne,” “epoxy” and “two-component” describe different aspects of a formulation. Hempel describes its two-component polyurethane and epoxy families as chemically crosslinking products. Confirm the actual product’s cure route. [Hempel mixing and thinning guide, January 2025](https://www.hempel.com/-/media/Hempel/Files/Technical-Guidelines/Application-Methods/Mixing--thinning--2026.pdf)

| Application route | Primary-source distinction | Selection question |
| --- | --- | --- |
| Air spray | Compressed air atomizes the liquid at the aircap | Can the approved coating and setup achieve the required appearance and coverage? |
| Airless spray | Fluid pressure drives atomization through an orifice | Is the system appropriate for the viscosity, build and production geometry? |
| Air-assisted airless | Fluid atomization is supplemented by air for pattern control | Does the selected setup balance finish, application rate and access? |
| Electrostatic application | Charged coating is attracted to the grounded target | Are the coating, equipment, grounding and part geometry suitable? |

Graco identifies finish quality, viscosity, object shape, transfer efficiency and production speed as equipment-selection variables. Its comparisons describe technologies rather than guaranteeing the same transfer efficiency on every part. Validate the actual load and equipment instead of copying a supplier's headline percentage. Brush/roller operations and local stripe coats must likewise be compatible with the selected product and project instructions. [Graco applicator technologies](https://www.graco.com/us/en/in-plant-manufacturing/solutions/articles/applicator-technology.html)

## Preparation, geometry and local coverage

Identify oils, soluble contamination, oxide, existing coatings, welds and the required surface condition before painting. Jotun’s **Penguard HB II application guide, 12 December 2024**, addresses contamination, weld/edge preparation and existing-coating compatibility. Confirm preparation for the actual substrate and system. [Jotun guide, first nine pages of linked combined file](https://www.jotun.com/api/v1/datasheets/download/merged?selectedFiles=9794%2C9761)

A drawing should identify significant surfaces, masking, no-coat regions, rack/contact marks and access inside cavities. Review sharp transitions, welds and recessed areas separately from broad faces. Jotun's guide includes stripe coating and warns against excessive local film build. A thicker visible face does not demonstrate coverage of a hidden return or repair a missed edge. [Jotun HB II guide, printed page 6](https://www.jotun.com/api/v1/datasheets/download/merged?selectedFiles=9794%2C9761)

## Mixing, working time and cure

Use the actual product's mixing basis and ratio. Do not interchange mass and volume ratios. Record component identity, batch, mix time, permitted induction period, material temperature, thinning and pot-life limit. Hempel warns that wrong mixing or thinner can change final performance; its guidance calls for the specified component ratio and recommended thinner. Adding solvent is not a substitute for adding the required curing component. [Hempel mixing guidance, pages 1–2](https://www.hempel.com/-/media/Hempel/Files/Technical-Guidelines/Application-Methods/Mixing--thinning--2026.pdf)

Keep touch-dry, handling, overcoating and service cure distinct. The Jotun HB II guide distinguishes touch dry from readiness for permanent exposure, and sets overcoating conditions by temperature and next-coat type. Its maximum interval can require further surface preparation when exceeded. These are separate controls; a part that can be handled is not automatically ready for immersion, assembly or the next coat. [Jotun HB II guide, printed page 7](https://www.jotun.com/api/v1/datasheets/download/merged?selectedFiles=9794%2C9761)

Log substrate/environmental conditions against the product's permitted window. Complete the required ventilation and curing conditions, then protect the surface against contamination and damage. Use the selected products' SDS and installed equipment instructions for mixing, spraying and cleaning; changing to a water-borne carrier does not itself define all handling controls.

## Wet film, dry film and an original calculation

Hempel relates required wet-film thickness to dry-film thickness and **solids by volume**, and explains that added thinner increases liquid volume without increasing coating solids. Use the as-applied mixture's value rather than substituting solids by mass. [Hempel calculation guidance, page 3](https://www.hempel.com/-/media/Hempel/Files/Technical-Guidelines/Application-Methods/Mixing--thinning--2026.pdf)

**Original hypothetical example:** assume a coating has **60% volume solids**, a target dry film of **120 µm**, additive liquid volumes and no application losses. Before thinning:

`required wet film = 120 / 0.60 = 200 µm`.

Now assume the product permits adding **10% solids-free thinner by volume of the original mixed paint**. This is an assumed permission, not a recommendation for a named product. One litre of mixed paint contains 0.60 L solids; after adding 0.10 L thinner, solids fraction is `0.60 / 1.10 = 0.54545`.

`required wet film = 120 / 0.54545 ≈ 220 µm`.

Verify cured dry-film thickness on the part. The model excludes roughness, overspray, evaporation before measurement, additional cure shrinkage and uncertainty.

## Inspection and production handoff

**ASTM D7091-22**, active with a catalog update of 23 June 2022, addresses nondestructive dry-film measurement of nonmagnetic coatings on ferrous metal and nonmagnetic, nonconductive coatings on nonferrous metal. Match the instrument/method to the coating and substrate; the standard's title does not support using every gauge on every material. Specify measurement locations, calibration/verification arrangements, sampling and acceptance separately from the nominal thickness. [ASTM D7091 public record](https://store.astm.org/standards/d7091)

Thickness, colour/gloss, local defects, adhesion, cure and corrosion performance answer different questions. Define the applicable tests and limits for the ordered system. Retain product/batch and mixing records, surface preparation, environmental and cure history, coat sequence, measured results and approved repair details. A smooth appearance or an acceptable thickness reading cannot by itself establish the other requirements.

Source dates are taken from the documents: Hempel's linked file says **January 2025** despite its “2026” filename. The combined Jotun download includes HB II dated **12 December 2024** followed by a separate HB guide dated **7 December 2015**; the examples here use the first document. Graco and the WBDG web discussion have no verified technical issue date assigned here. The worked values are original assumptions.

## Related reference

[Surface-finishing processes](index.md)