Coating Additives
Additives make up a small share of most formulas but solve outsized problems: foam, settling, poor flow, slow cure, UV damage, mildew and flash rust.
Key takeaways
- Additives are minor ingredients, commonly a few percent of the formula or less, that modify manufacture, storage, application or the final film.
- Each additive solves one problem but can create another; formulation is a balancing act verified by testing.
- Surface-active additives such as silicones can cause fisheyes and intercoat adhesion problems when misused.
- In the field, only use additives, accelerators or thinners approved on the product data sheet.
If binders, pigments and solvents are the main structure of a coating, additives are the fine adjustments. A few tenths of a percent of the right surfactant can turn a cratered, foaming product into one that sprays smoothly. Additives are usually grouped by the stage at which they act: during manufacture and storage, during application, and in the dry film.
Formulators select additives in combination and confirm the result with laboratory and field testing, because many additives interact. A defoamer that works in one resin may cause craters in another; a catalyst that speeds cure may shorten pot life to unworkable levels.
Manufacturing and storage additives
Wetting and dispersing agents
These surfactants and polymeric dispersants coat pigment particles, help the binder displace air from their surfaces, and keep them from re-agglomerating. Good dispersion improves color strength, gloss and stability, and reduces the binder demand of pigments and fillers.
Anti-settling agents
Dense pigments such as zinc dust and barytes sink during storage. Anti-settling agents build a weak structure in the can that holds particles in suspension but breaks down when stirred.
In-can preservatives and anti-skinning agents
Waterborne coatings can spoil from bacterial growth in the can, so they usually contain in-can biocides. Oxidatively drying alkyds tend to skin over in partially filled containers; volatile anti-skinning agents, of which methyl ethyl ketoxime has historically been the best known, delay that reaction until the film is applied.
Application and flow additives
Defoamers and deaerators
Mixing, rolling and pumping introduce air. Defoamers destabilize bubble walls so foam collapses, while deaerators help microbubbles rise and escape. They are important in waterborne products, thick self-leveling floors and roller-applied coatings. Too much defoamer, or the wrong type, can itself produce craters.
Rheology modifiers
Thickeners and thixotropes give a coating high viscosity at rest to resist sagging and settling, and lower viscosity under shear for spraying and brushing. Common types include fumed silica, organoclays, polyamide waxes, cellulosic thickeners and associative thickeners. The underlying behavior is explained in rheology and viscosity.
Flow, leveling and surface-control agents
Acrylic and silicone-based flow agents even out surface tension across the drying film, reducing orange peel, brush marks and cratering. Slip additives lower surface friction and improve mar resistance.
Silicone and fluorinated surface additives migrate to the film surface. Used at the wrong level, or carried into an area by aerosol sprays and lubricants, they can cause fisheyes and cratering and poor adhesion of the next coat. Keep silicone-containing products away from surfaces being prepared for coating.
Cure-related additives
Reactive coatings frequently use additives that control how fast and how completely the binder crosslinks. The chemistry is covered in depth in how coatings cure.
- Driers are metal carboxylates that catalyze oxidative crosslinking of alkyds and oils. Cobalt and manganese act mainly at the surface, while zirconium, calcium and similar “through” driers help the whole film cure. Concern over cobalt’s health classification has driven the development of alternative drier chemistries.
- Urethane catalysts such as organotin, bismuth and zinc compounds or tertiary amines accelerate the isocyanate–hydroxyl reaction.
- Epoxy accelerators, such as certain phenolic compounds and tertiary amines, speed amine cure, particularly at lower temperatures.
- Adhesion promoters, often organofunctional silanes, form chemical bridges between inorganic surfaces and organic binders.
Additives that protect the dry film
UV absorbers and light stabilizers
Sunlight breaks chemical bonds in many binders, leading to gloss loss and chalking and fading. UV absorbers, commonly benzotriazole or triazine types, convert UV energy into heat. Hindered amine light stabilizers (HALS) do not absorb UV but trap the free radicals produced by photodegradation. Combining the two is common in clear coats and exterior topcoats.
Dry-film biocides
Fungicides and algaecides slow mildew and algae growth on exterior and damp interior surfaces. They deplete over time by leaching and degradation, so they delay rather than permanently prevent growth.
Corrosion and flash-rust inhibitors
Waterborne coatings applied directly to steel can cause flash rusting as the water evaporates. Flash-rust inhibitors protect the steel during that window, while in-film corrosion inhibitors supplement anticorrosive pigments.
Coalescents and plasticizers
Coalescing solvents temporarily soften latex particles so they fuse into a continuous film, then evaporate. Plasticizers remain in the film to increase flexibility, but can migrate over time.
Additive classes at a glance
| Additive class | Main purpose | Possible side effect if misused |
|---|---|---|
| Wetting and dispersing agents | Stable pigment dispersion | Water sensitivity, foam |
| Defoamers and deaerators | Remove foam and entrained air | Craters, fisheyes |
| Rheology modifiers | Sag and settling control | Poor leveling, high spray pressure |
| Flow and slip agents | Smooth surface, mar resistance | Recoat adhesion problems |
| Driers and catalysts | Faster or more complete cure | Short pot life, wrinkling, brittleness |
| UV absorbers and HALS | Weathering resistance | Color interactions, reduced cure in some systems |
| Biocides | In-can or dry-film microbial control | Regulatory and handling restrictions |
| Flash-rust inhibitors | Prevent rust bloom under waterborne films | Water sensitivity at high levels |
Additives in the field
Applicators sometimes reach for additives to cope with difficult conditions: accelerators for cold weather, retarders for heat, or anti-slip aggregates for floors. Some manufacturers offer approved products for exactly these situations, listed on the product data sheet with defined dosages. Anything else is effectively reformulating the coating on site.
Record the batch numbers and quantities of any approved accelerator, retarder or thinner on the daily inspection report. If a problem appears later, that record is often the fastest way to rule additives in or out.
Frequently asked questions
Can I add a defoamer to fix bubbles in a floor coating?
Only if the manufacturer supplies or approves one. Bubbles in floors more often come from outgassing concrete, aggressive mixing or rolling technique, and an unapproved defoamer can cause cratering or adhesion loss.
Do UV stabilizers make epoxies suitable for sunlight?
Not in practice. Aromatic epoxies chalk and discolor in sunlight regardless of stabilizers, which is why they are normally topcoated with UV-resistant polyurethane, polysiloxane or fluoropolymer finishes outdoors.
Why do some coatings smell different from batch to batch?
Minor differences in solvents, coalescents or additive suppliers can change odor without affecting performance. A strong or unusual odor, however, is worth checking against the SDS and batch records.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
