Lacquer Coatings

A lacquer is a coating that forms a film mainly by solvent evaporation, leaving behind a resin that has not chemically crosslinked. Because the dry film can be redissolved by the solvents used to make it, lacquers are described as thermoplastic or non-convertible coatings. The term is also used loosely for fast-drying clear wood finishes, some of which are in fact catalyzed and do crosslink.

Film formation

Coatings form films in three broad ways (see film formation): solvent evaporation, coalescence of latex particles, and chemical reaction. True lacquers belong to the first group. High-molecular-weight polymer is dissolved in a strong solvent blend. When the solvents evaporate, polymer chains entangle and form a solid film. Nothing new is created chemically, so the film remains soluble.

This has two practical consequences:

• Fast dry and easy recoating. Each new coat partly dissolves the one below it, so layers “burn in” and fuse together. Sanding between coats is easy.
• Limited solvent and chemical resistance. The same solvents, and some household chemicals, can soften or remove the film.

Main types

Nitrocellulose lacquer

Nitrocellulose, made by nitrating cellulose, was the first widely used lacquer resin and transformed automotive and furniture finishing in the early twentieth century. It is usually combined with plasticizers and modifying resins. It sands well, rubs to a high gloss, and is still used on furniture, cabinets and musical instruments. It tends to yellow and can become brittle with age.

Acrylic lacquer

Thermoplastic acrylic lacquers offered better color and gloss retention than nitrocellulose and were used on cars for decades before being replaced by basecoat-clearcoat urethanes. Water-white acrylic lacquers are also used on wood and metal where non-yellowing clarity is wanted. See acrylic coatings.

Catalyzed lacquers

Pre-catalyzed and post-catalyzed lacquers, often based on nitrocellulose with urea- or melamine-formaldehyde resins and an acid catalyst, crosslink partially after application. They offer better durability than plain lacquer and are popular in cabinet shops. Strictly speaking they are not true lacquers, but the name persists.

Shellac

Shellac is a natural resin secreted by the lac insect and dissolved in alcohol. It dries by evaporation and is often grouped with lacquers. It is used as a sealer, a barrier coat over knots and stains, and a traditional French polish.

Other thermoplastic coatings

Chlorinated rubber and vinyl coatings also dry by solvent evaporation and behave like lacquers in many respects, though they are classed as industrial maintenance coatings.

Where lacquers are used

• Furniture, cabinetry and millwork (see coating wood)
• Musical instruments
• Clear protective coats on brass, copper and decorative metal
• Automotive refinish in historic restorations
• Nail polish and cosmetics (nitrocellulose)
• Paper, foil and packaging coatings

Application

Lacquers are usually sprayed because they dry too quickly to brush well. Conventional and HVLP spray guns are common in finishing shops. Several thin coats are applied rather than one heavy coat.

Thinning and retarders

Lacquer thinners are blends of fast and slow solvents. A retarder, which is a slow-evaporating solvent, may be added in hot or humid conditions to give the film more time to flow and to reduce blushing. See thinning coatings.

Spray booths and ventilation

Lacquers are typically high in solvents and flammable. Application should take place in a ventilated spray booth with fire precautions appropriate to flammable liquids (see fire and explosion hazards).

Common defects

• Blushing. A milky white haze caused by moisture condensing into the film as fast solvents evaporate and cool the surface. A retarder or warmer, drier conditions usually prevent it.
• Orange peel. Poor flow from incorrect thinning or gun setup (see orange peel).
• Cracking and checking. Old nitrocellulose finishes can craze with age and temperature cycling.
• Solvent pop. Trapped solvent forms bubbles if coats are applied too heavily.
• Fisheyes. Silicone contamination on the surface (see fisheyes and cratering).

Advantages and limitations

Advantages: very fast drying, easy repair and blending, excellent rubbing and polishing properties, and intercoat fusion.

Limitations: high VOC content, limited chemical and heat resistance, possible yellowing, and flammability. Many regions restrict lacquer VOC levels, and waterborne alternatives have become common (see VOC regulations).

Key terms

• Nitrocellulose. A cellulose-based resin that forms the classic furniture and instrument lacquer.
• Acrylic lacquer. A lacquer based on thermoplastic acrylic resin, with better color retention than nitrocellulose.
• Pre-catalyzed lacquer. A lacquer containing a catalyst that begins a light crosslinking reaction as the film dries, giving improved resistance.
• Post-catalyzed lacquer. A lacquer to which catalyst is added just before use, with a limited pot life.
• Blushing. A milky haze caused by moisture condensing in a fast-drying film in humid conditions.
• Burn-in. The way a fresh lacquer coat partly redissolves the previous one so the layers fuse.

Frequently asked questions

How can I tell whether an old finish is lacquer?

A common shop test is to rub an inconspicuous spot with lacquer thinner. Lacquer softens and becomes sticky quickly. Shellac softens with denatured alcohol. Crosslinked finishes resist both.

Can lacquer go over other finishes?

Lacquer solvents can lift or wrinkle many other finishes, including some oil-based paints. Test first or use a compatible barrier coat.

Is lacquer suitable for exterior use?

Plain lacquer generally performs poorly outdoors. Exterior work normally calls for crosslinked or UV-resistant systems.

Related entries

• Film formation
• Acrylic coatings
• Coating wood
• Solvents in coatings
• Alligatoring (new)
• Tack-free time and drying stages (new)