When a paint film dries leaving oriented parallel lines, the first reaction on site is usually that not enough leveling agent was added. Brush marks differ in origin from orange peel and craters: orange peel and craters mostly come from surface tension gradients, where a leveling agent can help; brush marks come more from the directional flow lines left by brushing and roller coating, and from insufficient leveling time caused by thixotropic structure recovering too fast and solvent evaporating too fast. Treating brush marks as a leveling problem often both wastes additive and hides the real cause.
Line Direction Matching the Application Direction Points to Rheology
The on-site features of brush marks are fairly easy to recognise: the line direction matches the brushing or roller coating direction and runs in parallel; they are visible right after application and gradually fade during drying but never disappear completely; and with the same formulation, switching to spray application makes the marks disappear or lighten noticeably, which shows the problem is related to application shear and the rheology of the system. Leveling of a coating needs two time windows: after application and before the thixotropic structure recovers, the film can flow; after the structure recovers, the film gains sag resistance. If thixotropic recovery is too fast, the film loses flow before it can level; if the solvent evaporates too fast, the surface dries while the lower layer is still wet.
Troubleshoot in the Order of Rheology, Solvent, Film Thickness
To deal with brush marks, first look at the dosage and combination of rheology additives and slow the thixotropic recovery rate somewhat; then look at the solvent evaporation gradient and adjust the proportion of slow solvent, bearing in mind that too much slow solvent lengthens drying time; then confirm film thickness, since a film that is too thin amplifies the marks.
Watch the Upper Limit When Choosing a Leveling Agent
Only after the first three steps should a leveling agent come into play. In brush mark problems it plays a finishing role, not the leading one. Acrylic types are compatible, rarely cause surface defects and suit improving leveling and feel; silicone types lower surface tension more markedly and suit solving wetting and craters, but carry a higher overdose risk, easily causing craters, haziness and reduced recoatability.
Chemitech's Solution
For film leveling, brush marks and surface defects, Chemitech offers acrylic and silicone leveling agent products.
Acrylic leveling agents suit solvent-based and water-based systems with a wide compatibility window, mainly improving leveling, feel and surface evenness; silicone leveling agents lower surface tension more strongly and are used to solve poor wetting and craters; for systems that need both, modified siloxane products are also available.
When selecting, it is best to establish first what the problem belongs to: surface tension problems call for silicone types, while insufficient flow and leveling call for acrylic types; using the two together requires controlling the ratio to avoid surface defects. Start dosage low and verify, since in most cases the leveling agent is an assistant rather than the main actor, and expecting a larger dose to solve brush marks usually brings limited results.
Customers can provide the current formulation and application conditions, and our technical team will help judge whether the problem lies in rheology or surface tension and determine whether the leveling agent needs adjusting. For suitable systems, see acrylic leveling agent solutions and coating leveling agent solutions.
Summary
Brush marks are usually not a leveling agent problem but the combined result of thixotropic recovery that is too fast and solvent evaporation that is too fast. Adjust the rheology additives first, then the solvent gradient, then confirm film thickness, and only then finish with a leveling agent; this order avoids a great deal of wasted trial and error. Chemitech can provide coating companies with leveling agent products and application technical support; for lines where surface defects keep recurring, judging the cause first is usually more effective than simply raising the additive dosage.