Determination of Drying Time of Coatings According to the Chinese National Standard GB/T 1728

Drying time is one of the most critical performance indicators for paints, coatings, and putties, directly affecting recoating intervals, production scheduling, and final film quality. The Chinese national standard GB/T 1728 establishes standardized laboratory procedures for determining the surface-dry and through-dry times of paint films and putty films under controlled conditions. This article explains the principle, sample requirements, test items, operating methods, and typical applications of GB/T drying time determination, helping coating manufacturers, contractors, and accredited third-party laboratories obtain reproducible and comparable results.

Technical Overview

Drying time refers to the period required for a wet coating film to transform from a liquid state into a solid, handleable film after application. The Chinese national standard GB/T 1728, "Determination of drying time of paint films and putty films," divides this process into two principal stages: surface drying and through drying (hard dry). Surface dry means the film surface can withstand light touching without transferring paint or exhibiting tack, while through dry means the entire film thickness has hardened sufficiently to resist pressure, cutting, and normal handling without damage.

GB/T 1728 is an empirical pass/fail standard. Simple physical operations, including finger touching, cotton-ball blowing, filter-paper pressure, cotton-ball pressure, and blade cutting, are performed at successive time intervals after application until the film passes. The shortest elapsed time from application to the first passing test is recorded as the drying time. Testing is carried out in the standard atmosphere specified in GB/T 9278, namely 23 ± 2 °C and 50 ± 5 % relative humidity, using test panels prepared in accordance with GB/T 9271 and samples obtained following GB/T 3186.

Comparable international methods include the ISO 9117 series (ISO 9117-3 ballotini surface-dry test, ISO 9117-4 mechanical drying recorder, and ISO 9117-6 print-free test), ISO 1517, ASTM D1640 for drying, curing, or film formation at room temperature, and ASTM D5895 for mechanical drying recorders. In China, the ballotini method is also standardized as GB/T 6753.2, providing an additional route for surface-dry evaluation.

• Dust-free stage: no dust adheres to the film surface
• Tack-free stage: the surface no longer feels sticky under light contact
• Surface-dry stage: passes finger-touch or cotton-ball blowing criteria
• Through-dry stage: passes pressure, blade, or thick-layer criteria
• Fully cured stage: ultimate hardness and solvent resistance achieved

Testing Samples

Waterborne Architectural Coatings

Interior and exterior emulsion wall paints are the most frequently tested category, because product standards such as GB/T 9755 and GB/T 9756 impose maximum surface-dry limits, typically no more than 2 hours. Samples are usually supplied as 0.5 to 1 liter of original, well-mixed material in sealed containers.

Solvent-Borne Industrial Coatings

Alkyd enamels, epoxy systems, polyurethane coatings, chlorinated rubber paints, and other solvent-borne products are tested to verify drying schedules declared in technical data sheets and to control batch-to-batch consistency of drier packages.

Wood Coatings and Varnishes

Nitrocellulose lacquers, polyurethane wood finishes, and clear varnishes are evaluated to determine sanding and recoating windows in furniture and joinery production lines.

Anticorrosive Coatings for Steel Structures

Zinc-rich primers, epoxy micaceous iron oxide intermediates, and polyurethane topcoats used on bridges, pipelines, and marine structures rely on accurate drying data to plan multicoat application sequences in the field.

Putties and Filler Pastes

Wall putties, automotive body fillers, and other thick-film materials are tested using the thick-layer drying method, since conventional thin-film procedures do not reflect their actual hardening behavior.

For all categories, films are applied to standard substrates such as tinplate panels, glass plates, or mild steel panels using film applicators, at the wet film thickness specified by the product standard or the manufacturer. Duplicate panels are prepared so that separate areas can be checked at each interval.

Testing Items

• Surface drying time by the finger-touch method
• Surface drying time by the cotton-ball blowing method
• Surface drying time by the ballotini (small glass ball) method, GB/T 6753.2
• Through drying time by the filter-paper pressure method
• Through drying time by the cotton-ball pressure method
• Through drying time by the blade cutting method
• Thick-layer drying time of putty films
• Complete drying-stage profile using a linear mechanical drying-time recorder
• Print-free and stacking resistance evaluation, based on the principle of ISO 9117-6

Testing Methods

The general procedure is identical for all methods. The film is applied at the specified thickness, timing begins immediately after application, and the panels are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity. Pass/fail checks are then performed on fresh, untouched areas at progressively longer intervals, for example 15 minutes, 30 minutes, 1 hour, 2 hours, and so on. The result is expressed in hours as the shortest interval at which the film passes the chosen criterion, and the specific method used must always be stated in the report.

Finger-Touch Method (Surface Dry)

A clean finger lightly touches an untested area of the film for a brief moment. The film passes if no paint transfers to the finger and no visible indentation or mark remains on the surface. This is the simplest and most commonly used surface-dry check.

Cotton-Ball Blowing Method (Surface Dry)

A small ball of clean, degreased absorbent cotton of approximately 1 cubic centimeter is placed on the film surface. At the selected interval, a short, directed horizontal stream of air is blown at the ball. The film is judged surface dry if the ball is removed and no cotton fibers remain adhered to the surface.

Filter-Paper Pressure Method (Through Dry)

A square of qualitative filter paper, typically 20 mm by 20 mm, is laid smoothly on the film, and a drying-test weight delivering a 200 g force over a 1 square centimeter contact area is applied for 30 seconds. After the weight is removed and the panel is inverted, the film is judged through dry if the paper falls off freely or can be lifted away without tearing, lifting the film, or leaving attached fibers other than loose fibers that can be lightly dusted off.

Cotton-Ball Pressure Method (Through Dry)

A cotton ball of about 1 cubic centimeter is placed on the film and loaded with the 200 g drying-test weight for 30 seconds. The film passes if, after removal of the weight and ball, no cotton fibers adhere to the surface and no glossy pressure impression remains.

Blade Method (Through Dry)

A sharp knife blade is used to cut the film cleanly down to the substrate. The film is judged through dry if no coating sticks to the blade and the underside of the cut film shows no tacky or wet condition, confirming that hardening has occurred through the full film thickness.

Thick-Layer Method for Putty Films

The putty is applied at the thickness specified in the product standard, and at each interval a portion is lifted or cut with a spatula to expose the inner material. The putty is judged through dry when the interior no longer shows wetness or plastic tack.

Mechanical Drying-Time Recorder (Optional)

Following the principles of ASTM D5895 and ISO 9117-4, a stylus or needle travels slowly along a coated strip over a period such as 24 hours. The trace transitions from a complete film tear, to film cutting, to a surface track, and finally to an invisible line, allowing the dust-free, tack-free, dry-hard, and dry-through points to be read directly from the strip. This method is particularly valuable in research and development for comparing formulations.

Applications

• Factory quality control and batch release of architectural, industrial, and wood coatings
• Compliance verification against product standards such as GB/T 9755 and GB/T 9756 for emulsion wall paints
• Construction scheduling, including recoating intervals, walk-on times, and demasking windows
• Steel structure, pipeline, and marine anticorrosion projects requiring defined overcoat timing
• Automotive refinish and OEM production line takt-time planning
• Formulation research on drier dosage, solvent balance, and resin performance
• Tender documentation, product certification, and import and export inspection performed by accredited third-party laboratories

FAQ

What is the difference between surface dry and through dry? Surface dry means the outer skin of the film has lost its wet tack and passes gentle touching, while through dry means the entire film thickness has hardened and can withstand pressure, cutting, and handling without damage or paint transfer.

Which Chinese national standard governs drying time determination? GB/T 1728 is the primary national standard for determining the drying time of paint films and putty films. GB/T 6753.2 provides an additional surface-dry test using small glass balls, aligned with the principle of ISO 1517.

Under what conditions are the tests performed? Tests are conducted at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with GB/T 9278, using panels prepared per GB/T 9271, because temperature, humidity, and film thickness all strongly influence drying behavior.

How are results expressed? Results are reported as the time in hours from application to the first passing observation, together with the exact method used, the film thickness, the substrate, and the conditioning atmosphere. Duplicate determinations should agree closely, and the arithmetic mean is normally reported.

Are GB/T results comparable with ISO or ASTM results? The underlying principles are closely related: the ballotini test corresponds to ISO 9117-3, and the mechanical recorder corresponds to ISO 9117-4 and ASTM D5895, while ASTM D1640 covers similar staged drying evaluation. Exact figures should nevertheless be compared with caution because pass criteria and apparatus differ slightly between standards.

What factors can prolong drying time? Low temperature, high humidity, excessive film thickness, poor ventilation, retarding solvents, insufficient driers in alkyd systems, and highly absorbent or cold substrates are the most common causes of extended drying times.

How much sample is required for a test? For liquid coatings, approximately 0.5 to 1 liter of well-mixed original material is generally sufficient to prepare all required panels, including duplicates and retentions.