Serrated Drawdown Test for Coating Leveling Properties: Principle, Standards, and Procedure

The serrated drawdown test, also known as the comb-type or notched-applicator leveling test, is a widely used laboratory method for evaluating the flow and leveling capability of liquid coatings. By drawing a wet film with a notched blade that leaves parallel ridges of controlled geometry, the test reveals how effectively a coating smooths out surface striations before it dries. This article explains the technical principle, applicable standards, sample requirements, operating procedure, rating criteria, and typical applications of the sawtooth drawdown leveling test.

Technical Overview

Leveling is the ability of an applied wet coating film to flow out and eliminate surface irregularities such as brush marks, roller stipple, and application striations before the film solidifies. Poor leveling produces visible ridges, non-uniform gloss, and reduced aesthetic and protective performance; excessive flow, on the other hand, leads to sagging on vertical surfaces. The serrated drawdown test was developed to quantify this property under controlled, repeatable laboratory conditions.

In this method, a leveling test blade—a serrated or comb-type applicator whose working edge carries a series of paired teeth or grooves of decreasing film thickness—is drawn across a test chart. The notches deposit parallel ridges (sawtooth striations) of known geometry in the wet film. As the coating flows under the driving force of surface tension, adjacent ridges progressively merge. After a prescribed interval, the operator inspects the film and rates the degree of flow-out: the more ridge pairs that have fused into a smooth, continuous surface, the better the leveling of the coating.

The underlying physics involves capillary-driven flow, in which surface tension acts to minimize the film's surface area while viscous resistance and thixotropic structure recovery oppose it. Key influencing factors include low-shear viscosity, rheology modifier type and dosage, solvent evaporation rate, wet film thickness, surface tension gradients, and substrate absorbency. Relevant method and product standards include ASTM D2801 (draw-down test for leveling of liquid paints), ASTM D4062 (leveling of architectural paints by draw-down), ASTM D823 (production of uniform-thickness films), and ASTM D4400 (sag resistance by notch-coated rods) as a complementary flow-behavior test. Chinese architectural coating product standards such as GB/T 9755 and GB/T 9756 also list leveling as a required technical indicator, typically assessed against a standard sample.

Testing Samples

Waterborne Architectural Coatings

Interior and exterior emulsion wall paints, primers, sealers, and textured masonry coatings are the most frequently tested materials. These products are applied by roller or brush in the field, so their ability to flow out roller stipple and brush marks directly determines the final appearance of walls and ceilings.

Solvent-Borne and Industrial Coatings

Alkyd enamels, epoxy and polyurethane systems, high-solids coatings, and coil coating or automotive refinish topcoats can be evaluated by the same principle. For industrial products, the test helps verify that spray- or roller-applied films achieve uniform gloss without orange-peel-like striation defects.

Wood Coatings and UV-Curable Systems

Nitrocellulose lacquers, polyurethane wood finishes, waterborne furniture coatings, and UV-curable coatings are tested to confirm that application marks flow out before flash-off and curing, especially for high-gloss and clear finishes where surface defects are highly visible.

Raw Materials and Formulation Intermediates

Formulators use the test to benchmark rheology modifiers (HEUR and HASE thickeners, cellulose ethers), leveling and slip agents, dispersants, and co-solvent packages. Sample requirements for all categories include:

• Representative sampling of the liquid paint, mixed thoroughly to uniform consistency without entraining air bubbles.
• Conditioning of the sample at 23 ± 2 °C and 50 ± 5 % relative humidity for a defined period before testing.
• Clean, flat substrates such as sealed drawdown charts, lacquered paper cards, glass plates, or smooth metal panels, consistent between comparative tests.
• Specification of whether the sample is tested as supplied, after defined aging, or diluted to application consistency.

Testing Items

The core deliverable of the test is a leveling rating that characterizes the flow-out behavior of the wet film. Depending on the adopted method standard and the client's specification, the reported test items typically include:

• Leveling rating or leveling index, expressed as the number of ridge pairs that have visibly flowed out and merged, or as a classification matched against pictorial standards.
• Flow-out observation at one or more defined time intervals (for example, 5 min and 10 min after drawdown) to capture time-dependent behavior.
• Gloss uniformity of the dried film, optionally measured with a 60° or 85° glossmeter across the ridge area to provide an objective numerical supplement to visual rating.
• Surface appearance description, including residual striations, cratering, or surface defects observed during flow-out.
• Paired sag-resistance data obtained with a notch-coated rod method, where the balance between leveling and sagging must be documented for vertical applications.
• Reference comparison, where the sample is rated against a retention sample or standard material tested in parallel under identical conditions.

Testing Methods

Apparatus and Materials

The test requires a leveling test blade (serrated applicator) with pairs of notches that produce wet film ridges of stepped thickness, typically spanning a range appropriate to the coating type (for example, from approximately 100 µm to 500 µm). An automatic film applicator or motorized drawdown machine ensures a uniform, reproducible application speed, as prescribed by the film-preparation practice in ASTM D823. Additional items include sealed drawdown charts or glass panels, a leveled flat test bed, a timer, and, where required, a calibrated glossmeter and a light source for oblique illumination of the ridge pattern.

Sample Preparation and Conditioning

Samples are conditioned to the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity. Each sample is homogenized with slow stirring to avoid air entrapment, and any skin or contamination is removed. Substrates are conditioned in the same environment so that temperature differences do not distort the flow behavior of the wet film.

Drawdown Procedure

The test chart is secured on the flat bed, and a sufficient quantity of the coating is placed ahead of the serrated blade. The blade is drawn at a constant speed in a single continuous stroke, depositing a wet film containing parallel sawtooth ridges of stepped height. The panel is immediately placed horizontally in a dust-free location, and timing starts at the completion of the drawdown. The film is left undisturbed so that flow-out occurs under quiescent conditions, closely simulating the behavior of a freshly applied coating on a wall or workpiece.

Evaluation and Rating

At the time interval specified by the selected method, the ridge pattern is examined under diffuse and oblique lighting at a defined viewing angle. The rating is assigned by one of the following approaches:

• Counting the ridge pairs that have completely flowed out and merged into a smooth surface, with a higher count indicating better leveling.
• Matching the observed flow-out pattern against photographic or pictorial standards supplied with the method, reporting the closest classification.
• Measuring the gloss of the leveled area after drying and comparing it with the gloss of a smooth reference area of the same film thickness.

Tests are normally performed in duplicate or triplicate, and the report includes the leveling rating, blade geometry and wet film thickness, drawdown speed, ambient temperature and humidity, substrate type, and observation time. For acceptance purposes, the result is compared with the specification limit or with a reference sample evaluated in the same session.

Applications

The serrated drawdown leveling test is used throughout the coatings value chain for formulation, quality control, and compliance purposes:

• Formulation development: optimizing the balance among rheology modifiers, co-solvents, and leveling additives to achieve smooth flow-out without sagging, particularly in low-VOC waterborne systems where low-shear viscosity is high.
• Quality control: verifying batch-to-batch consistency of architectural paints and industrial coatings before release, and investigating customer complaints related to brush marks, roller stipple, or poor appearance.
• Product certification: demonstrating compliance with the leveling requirements of architectural coating product standards and with purchaser technical specifications in tenders and supply agreements.
• Application research: comparing leveling performance across application methods (brush, roller, spray) and evaluating wet-on-wet multicoat systems where underlying striations must be covered by subsequent layers.
• Raw material evaluation: benchmarking thickeners, flow and leveling agents, and defoamers from different suppliers under identical drawdown conditions.
• Specialty coatings: assessing self-leveling floor coatings, high-build protective systems, and clear wood finishes where optical smoothness is a primary quality attribute.

FAQ

What is the difference between leveling and sagging? Leveling is the flow of a wet film that removes surface irregularities on horizontal or low-slope surfaces, whereas sagging is undesired gravity-driven flow that causes the film to run on vertical surfaces. The two properties are opposing requirements: a coating that levels very easily often sags more readily, so formulators use the serrated drawdown test together with a sag-resistance test to find the optimum balance.

How is the leveling rating interpreted? A higher rating—more ridge pairs flowed out, or a closer match to the smoothest pictorial standard—indicates better leveling. Acceptance limits depend on the product specification; high-gloss decorative finishes generally demand the highest ratings, while flat or textured coatings tolerate lower values.

Which wet film thickness should be used? The blade or gap setting should approximate the real-world application thickness of the coating. Testing at unrealistically high thickness will overstate leveling, while very thin films will understate it, so comparative evaluations must always use identical geometry.

Do temperature and humidity affect the result? Yes. Higher temperature lowers viscosity and accelerates solvent evaporation, while high humidity slows water release from waterborne paints. All tests should therefore be conducted and reported at the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity.

Why do waterborne paints often show lower ratings than solvent-borne paints? Emulsion paints rely on associative thickeners that build viscosity at low shear, and water evaporates more slowly with a stronger humidity dependence, both of which restrict flow-out. Modern low-VOC formulations close this gap through careful selection of rheology modifiers and coalescing agents.

Can powder coatings be tested with this method? No. The serrated drawdown test applies only to liquid coatings. Powder coatings are evaluated for flow and smoothness after melt and cure using dedicated methods such as inclined-plate melt flow or cured-film smoothness comparison panels.

How can result variability be minimized? Use an automatic applicator for constant drawdown speed, condition all materials and substrates, avoid air bubbles in the sample, use the same substrate type for comparative tests, and evaluate replicate panels under standardized lighting and viewing geometry.