Concept · Conditions and application

Drying, curing and recoat windows

Touch dry is not cured. How temperature and humidity change cure, what a recoat window is, and how inspectors check cure.

A recoat window opens after a minimum time and closes at a maximum. Cold steel moves both limits later.

A coating dries when its solvent leaves and cures when its binder reacts or forms its final film. A film can be dry to touch long before it is cured.

How a film forms

Four routes lead to a finished film. The route decides what can go wrong.

Physical drying (vinyl, acrylic): the solvent evaporates and the binder stays unchanged.
Oxidative curing (alkyd, drying oils, epoxy ester): the binder reacts with oxygen from the air.
Chemical curing (epoxy, polyurethane, polyester, vinyl ester, silicate, polysiloxane): the binder molecules react and join into a network. Two-pack paints react after mixing. Silicates and some polysiloxanes react with moisture from the air.
Coalescence (water-borne paints): the paint particles fuse as the water leaves. High RH slows this.

Two clocks run in a chemically cured film. Evaporation of solvent sets the touch-dry time. The chemical reaction sets the cure. A surface can feel dry while the reaction is only part way through.

Temperature drives both clocks. As a rule of thumb, a chemical reaction doubles its rate for each 10 °C rise, and it halves for each 10 °C fall. Cold steel therefore stretches the cure. A data sheet also gives a minimum cure temperature. Below it, the cure can stall.

Moisture can help or harm, depending on the chemistry.

Ethyl silicate zinc needs moisture from the air. Very dry air slows its cure (zinc-rich primers).
Isocyanate in polyurethane also reacts with water. The reaction releases carbon dioxide, and the gas can leave bubbles or pinholes in the film.
In an amine-cured epoxy, free amine at the surface reacts with moisture and carbon dioxide. It forms a sticky, waxy film called amine blush. Cold and humid air raise the risk. The film breaks the bond to the next coat.

The recoat window

The data sheet gives a minimum and a maximum time before the next coat, at stated temperatures. Both limits have a reason.

Too early: the first coat has not released its solvent. A new coat can seal the solvent in, and later it makes bubbles (solvent entrapment). The first coat also needs time to gel. If it has not gelled, the new coat can make it sag or run.

Too late: the first coat keeps cross-linking. The network gets denser and the surface harder. The next coat mixes less well into it, so the bond between the coats is weaker. Many coatings then need abrading before the next coat. Dust and salts on an aged film add a weak layer between the coats.

Cold weather moves both limits later. Use the steel temperature during the cure to pick the row of the data sheet. The air temperature at the time of application does not decide it.

In the field

Note the time the first coat finished. Read the recoat interval on the data sheet for the temperature the steel had since then.
Check the temperature and RH records. Look for cold or damp periods in the cure.
Look and feel for contamination. A waxy, oily or sticky film means amine blush. Dust, salts and oil also stop the bond.
Check cure with the test that the specification names. For ethyl silicate zinc, use the MEK rub test (ASTM D4752) and agree the required rating first. For organic coatings, use solvent rubs (ASTM D5402), a hardness check or the data sheet's own test.
If the maximum time is past, hold the work. Record it and ask the engineer and the coating maker for the repair method.
If blush is present, wash with detergent and water, rinse and let the surface dry. Abrade if the data sheet or the maker asks for it.

The MEK rating runs from 0 to 5. A rating of 5 means no zinc on the cloth after 50 double rubs, and it usually allows topcoating. Confirm this with the coating maker.

A worked example

An example data sheet for an epoxy intermediate coat gives these intervals. At 20 °C the recoat interval is 12 hours to 7 days. At 5 °C it is 36 hours to 21 days. The first coat finishes on Monday at 16:00. The steel stayed near 5 °C.

The earliest overcoat time is Monday 16:00 + 36 hours = Wednesday 04:00. An inspector who checks on Tuesday at 14:00 finds 22 hours have passed, so the work waits. At Wednesday 08:00, 40 hours have passed. The coat is inside the window.

Check: Monday 16:00 + 24 hours = Tuesday 16:00, and 12 more hours = Wednesday 04:00.

What the inspector records

The time each coat finished and the time of overcoating.
The steel and air temperatures and the RH during the cure.
The data sheet interval that was used.
The cure test and its result, and the blush check.
Any cleaning, abrading or repair before the next coat.

Exam traps

Trap: treating touch dry as cured.
Trap: reading only the minimum interval. The maximum matters too.
Trap: using the 20 °C row of the data sheet in cold weather.
Trap: believing that waiting longer is always safe. Past the maximum, the next coat can fail to bond.
Trap: skipping the blush check because the surface looks dry.
Trap: thinking dry air helps every coating. Ethyl silicate zinc needs moisture to cure.

How the exams treat it

The FROSIO book covers the drying and curing routes above and the use of data sheets. See also pot life for the first hours after mixing.

Use it: Defect: amine blush

207 practice items in the members area mention this topic (CIP 1: 77 · CIP 2: 81 · FROSIO written: 49).

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

About this page

Sources: ASTM D4752-20(2024) (MEK rub test, ethyl silicate zinc) · ASTM D5402-19(2024) (solvent rubs, organic coatings) · ISO 12944-5:2019, cl. 6.2 and Table A.1 · KTA-Tator, The Formation of Amine Exudate (Blushing) of Epoxy Coatings and Common Causes of Blistering and Bubbling in Industrial Coatings · Bannister, Durham University PhD thesis, section 2.3.4 (overcoating interval in epoxy coatings) · FROSIO Surface Treatment Book (drying and curing types).

Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.