Two-component polyurethane paints

Hempel's Hempathane HS 55610 mixes base 55619 to curing agent 97050 at 7:1 by volume. At product temperature, pot life is 3½ hours at 10°C, 2 hours at 20°C and 1 hour at 30°C. Its dry-film range is 50–125 µm. Check the separately stated dew-point, humidity, recoat and service-cure requirements. Hempathane HS 55610 PDS, October 2026

Specify the resin and curing agent as a two-pack system. Mixed working life, film cure, weather resistance and application controls depend on the named product.

Resin and crosslinker families

Conventional polyurethane chemistry reacts hydroxyl-functional polyols with polyisocyanates. Covestro describes polyester, polyether and polycarbonate polyols and distinguishes aromatic and aliphatic isocyanate building blocks. Its blocked polyisocyanates serve one-component baking systems, which should not be confused with an ambient-curing two-pack paint. Covestro polyurethane coating chemistry

An “acrylic polyurethane” can use an acrylic polyol with an isocyanate crosslinker; it does not mean an ordinary acrylic paint with a universal hardener added. For example, Covestro identifies Desmodur ultra N 3700 as an aliphatic, highly functional HDI-trimer hardener, with polyacrylate or polyester polyols as preferred coreactants for lightfast coating systems. This is a raw-material example, not a recipe for modifying a finished paint. Covestro Desmodur ultra N 3700

Waterborne and solventborne formulations also require separate selection. BASF lists waterborne acrylic-polyol dispersions for 2K polyurethane primers and topcoats alongside polyester-polyol and polyurethane-dispersion technologies. The carrier does not identify the crosslinker or eliminate the need to read its SDS. BASF Basonol technologies

Mixing, induction and pot life

Use the specified base, hardener and thinner. A ratio stated by volume is not the same numerical ratio by mass: component densities matter. Do not compensate for an incorrect ratio by adding more thinner or extra curing agent. Record the product identities, ratio basis, mixing time, material temperature and time of mixing.

International's Interthane 990 instructions require mixing complete supplied units: agitate Part A, add the entire Part B and mix thoroughly. The stated ratio is 6:1 by volume. The sheet prohibits leaving mixed material in hoses or spray equipment during stoppages and recommends fresh units after prolonged interruptions. These instructions belong to this product, not to all polyurethane paints. Interthane 990 TDS, 26 May 2026

Induction or sweat-in is any required waiting period after mixing and before application. Pot life is the permitted working interval of the mixed paint under stated conditions. Neither is the drying time of an applied film. Sherwin-Williams Polylon HP, for example, specifies 2:1 by volume, no sweat-in, and 4 hours pot life at 25 °C in its schedule at 50% RH. Its pot life falls to 45 minutes at 38 °C. Do not invent an induction delay where none is required or transfer these times to another grade. Polylon HP data sheet, revised 20 November 2023

Control moisture, substrate temperature and film build

A dry-looking surface can still be near the dew point. Hempel's Hempathane HS 55610 requires a clean, dry surface at least 3 °C above dew point, relative humidity below 85% during application and curing, and protection from rain or condensation during its stated early interval. Its curing agent is moisture sensitive and must be kept tightly closed in dry storage. These are product-specific controls, not permission to apply at any temperature whenever the air feels dry. Hempathane HS 55610 PDS, October 2026

For Hempathane HS 55610, 80 µm dry is recommended. Keep build within the product range and apply its separate temperature and moisture controls. Hempathane HS 55610 PDS, October 2026

Measure wet film during application and dry film after drying using the specified procedure. Check edges, welds, recesses and areas applied by brush or roller rather than assuming that nominal coverage proves local film thickness. Include approved thinning when setting wet-film targets; theoretical coverage does not include production losses.

Recoat readiness is different from service cure

Jotun Hardtop XP provides a bounded example of distinct milestones. Its US TDS specifies a typical 50–100 µm dry film and a 10:1 volume ratio of Comp A to Comp B. At 23 °C substrate temperature, it reports:

The drying data apply under controlled temperatures, RH below 85%, and at the average of the product's dry-film range. “Cured for service” means the minimum time before permanent exposure to the intended environment; a touch-dry surface is insufficient. Hardtop XP US TDS, 24 June 2026

An extended recoat window does not remove surface preparation. Jotun's application guide requires checking compatibility and the underlying product's overcoat interval, and removing contamination and loose or chalked coating. Its extended-overcoat guidance remains conditional on good painting practice and actual exposure; aged, chalked or contaminated surfaces can require abrasion and cleaning. Hardtop XP application guide, 29 October 2026

Qualify the complete primer and exposure system

A finish coat is not automatically a direct-to-metal corrosion system. Interthane 990 requires a recommended anticorrosive coating scheme and application within the underlying coat's overcoat limits. It is not recommended for immersion; severe chemical or solvent splashing requires consultation about suitability. Outdoor gloss retention therefore does not establish tank-lining or chemical-immersion performance. Interthane 990 TDS, 26 May 2026

Compare tests only with their substrate and layer construction. Polylon HP's reported 63 mg average abrasion loss uses ASTM D4060, a CS17 wheel, 1,000 cycles and 1 kg load. Its default tested system is blast-cleaned steel with 150 µm Macropoxy 646 and 75 µm Polylon HP, unless otherwise noted. That result is not a universal polyurethane wear rate or a prediction for another primer and film thickness. Polylon HP data sheet, revised 20 November 2023

Hardener handling follows the actual SDS

The US Hardtop XP Comp B SDS, revised 27 January 2026, identifies HDI oligomers and residual hexamethylene diisocyanate. It classifies the supplied mixture as flammable, harmful if inhaled, capable of causing an allergic skin reaction and respiratory irritation. It requires adequate ventilation and describes process enclosure or local exhaust controls; respirator selection must address exposure and operate within a respiratory-protection program with fitting and training. Gloves and eye protection also depend on the task and assessment. These instructions apply to this supplied hardener; review the base and thinner SDS as well, including mixing, spraying and cleanup. A cured-film property or low-VOC description does not establish the handling requirements of the uncured components. Jotun Hardtop XP Comp B US SDS, version 2.02

For procurement and qualification, retain the complete layer specification, current local TDS/SDS, measured film thicknesses, environmental conditions, mixing record and cure history. Verify the required appearance, adhesion and exposure performance on that construction. See Surface-finishing processes for the wider process context.