Direct-to-Metal (DTM) Coatings

A direct-to-metal (DTM) coating is a paint formulated to adhere directly to prepared metal without a separate primer, while also providing the appearance and weathering performance of a finish coat. DTMs are sometimes called primer-finishes or self-priming topcoats. They are popular for light industrial maintenance, equipment, structural steel in mild environments, and architectural metalwork where fewer coats save time.

How DTM coatings work

A traditional system uses separate layers, each with a job (see anatomy of a coating system): a primer for adhesion and corrosion inhibition, and a topcoat for weathering and appearance. A DTM combines these roles in one formulation:

• Adhesion promoters and wetting resins help the film bond to metal.
• Corrosion-inhibiting pigments, such as zinc phosphate or other non-toxic inhibitors, slow corrosion at the interface.
• Weather-resistant binders and pigments give color and gloss retention.

Most DTMs are applied in one or two coats of the same product.

Common chemistries

Waterborne acrylic DTM

The most common type for architectural and light maintenance work. These products are low in odor and VOC, dry quickly, and resist UV well. They are used on handrails, doors, frames, light structural steel and galvanized items. See acrylic coatings.

Alkyd and modified alkyd DTM

Solventborne alkyds wet marginal surfaces well and are tolerant of less-than-perfect preparation. Quick-dry and silicone-modified versions improve speed and gloss retention. See alkyd coatings.

Epoxy DTM

Some epoxy and epoxy mastic products are designed for direct application to steel with tolerance for hand- or power-tool cleaned surfaces. They are usually overcoated outdoors because epoxies chalk in sunlight.

Polyurethane and polyaspartic DTM

Certain two-component urethanes and polyaspartic coatings are formulated to go directly onto blasted steel, sometimes as single-coat systems for fast turnaround in fabrication shops.

Where DTM coatings fit

DTMs are generally suited to mild and moderate environments, roughly the lower ISO 12944 corrosivity categories (see ISO 12944 corrosivity categories). Typical uses:

• Interior and sheltered exterior steel
• Fences, railings, doors and frames
• Agricultural and light industrial equipment
• Shop-applied finishes on fabricated parts
• Galvanized items needing color

For severe service such as immersion, marine splash zones or chemical plants, multi-coat systems with dedicated primers are usually specified instead.

Surface preparation

“Direct to metal” does not mean “no preparation.” Performance depends heavily on the surface:

• Remove oil and grease by solvent cleaning or detergent washing.
• Remove loose rust and mill scale by hand tool cleaning, power tool cleaning, or blasting, as the data sheet requires.
• Treat new galvanized steel according to the manufacturer’s instructions, which may include cleaning and light abrading.
• Prime bare spots on previously coated surfaces if the data sheet requires spot priming.

Many DTM data sheets give different performance expectations depending on the preparation grade. Better preparation generally gives longer service.

Application tips

• Film thickness. DTMs rely on adequate thickness for barrier protection. Measure wet film during application (see wet film thickness measurement).
• Edges and welds. Apply a stripe coat on edges and welds where the film naturally thins.
• Flash rust. Waterborne DTMs on bare steel can show flash rust if applied in cool, humid conditions or too thin. Many formulations include flash-rust inhibitors; follow the minimum temperature and humidity limits.
• Two coats over one. Two thinner coats of DTM often perform better than one heavy coat because they reduce pinholes and missed spots.

Advantages and limitations

Advantages:
– Fewer products to stock and fewer coats to apply.
– Faster project turnaround.
– Many waterborne options with low VOC.

Limitations:
– Usually lower corrosion resistance than a dedicated primer-and-topcoat system in aggressive environments.
– Performance is sensitive to surface preparation.
– Not all DTMs are suitable for galvanized, aluminum or stainless steel; check the data sheet.

Specifying DTM coatings

A clear specification includes:

1. The generic type and named product.
2. The surface preparation standard (see SSPC/NACE surface prep standards).
3. The number of coats and dry film thickness per coat.
4. Ambient condition limits during application and cure.
5. Inspection points and acceptance criteria.

Key terms

• DTM. Direct to metal, meaning a coating formulated to be applied without a separate primer.
• Primer-topcoat (or unicoat). Another name for a single product that serves as both primer and finish.
• Flash rust. Light rust that forms on freshly cleaned steel, particularly under waterborne coatings (see flash rust).
• Inhibitive pigment. A pigment, such as a phosphate-based compound, that slows corrosion at the metal surface.
• Surface tolerance. The ability of a coating to wet and bond to less-than-ideal surfaces, such as tightly adherent rust.
• Edge retention. How well a coating maintains thickness over sharp edges as it dries.

Frequently asked questions

Is a DTM the same as a primer?

No. A DTM is designed to serve as both primer and finish. A primer alone is not usually designed for long-term exposure.

Can a DTM go over rust?

It can go over tightly adhered rust if the data sheet allows it, after loose rust is removed. Rust converters are a separate product type (see rust converters).

Can a DTM be topcoated later?

Usually yes, with a compatible product. Follow the recoat guidance.

Related entries

• Anatomy of a coating system
• Acrylic coatings
• Coating carbon steel
• Epoxy mastic
• Wash primers and etch primers (new)
• Rust converters (new)