Curing conditions and time-to-use

Overview

"Dry" and "cured" are different milestones. A finish is dry to the touch when the surface skins over, recoatable when a fresh coat will bond without trapping solvent or moisture, and cured when the film has finished hardening and reaches its rated hardness and chemical resistance. The makers' own data sheets list these as separate numbers, and the gap between them is large: hours for the first, weeks for the last. Every figure below is the manufacturer's, measured at stated conditions, and none is a guarantee for your shop.

When to use

• Planning a schedule: how long before the second coat, before sanding, before the piece sits in a room.
• Deciding whether a cold, damp or hot shop will cost you a coat.
• Not a substitute for the label on your can; formulations change.

Materials and tools

• A thermometer and a hygrometer in the finishing space.
• The product's data sheet.
• A cotton ball and 400 grit for the dry test described below.

Steps

1. Read the conditions behind the numbers. Minwax states its Wipe-On Poly times at 77F and 50% relative humidity; General Finishes quotes 70F and 50%. Cooler, more humid, stagnant air or heavy coats all lengthen them (Minwax; General Finishes).
2. Plan around these published times (maker figures, not tested by us):

Product	Touch	Recoat	Light use	Full cure
Minwax Wipe-On Poly (oil-based)	not listed	2 h; sand after 2-3 h	final coat 24 h	not listed
Minwax Polycrylic Max (water-based)	1 h	2 h	24 h	not listed
General Finishes Arm-R-Seal (oil-based)	12+ h	24+ h	7-10 days	30 days
General Finishes High Performance (water-based)	30+ min	2+ h	7-10 days	21 days
Osmo Polyx-Oil (hardwax oil)	not listed	8-10 h	not listed	2 weeks

3. Stretch the times when conditions are poor. General Finishes says to add time if humidity is over 80%, it is cool, three or more coats are on, or coats are thick. Woodcraft suggests working at roughly 65-80F: cold slows drying, while heat can dry a coat before it levels.
4. Test, don't guess. General Finishes says an oil-based coat is dry when it is no longer tacky, feels cool to the touch, and a pressed cotton ball leaves nothing stuck. For its water-based finish the test is that it sands to a dry powder with 400 grit or a 220-320 foam pad.
5. Keep the cure period light. Until the cure time passes, avoid placing hot dishes, rugs, plastic mats or wet cloths on the surface. General Finishes advises 21 days for water-based and 30 days for oil-based finishes before cleaning; Osmo gives 2 weeks before damp mopping or rugs.

Common problems

• Blush (milky haze in lacquer or shellac). Cause: moisture condensing as fast-evaporating solvent chills the film. Popular Woodworking says it shows on humid days and often fades by the next day. Fix: wait, mist retarder or blush eliminator, or rub with fine steel wool. A Finishing Forum reply on WoodWeb advises not spraying lacquer or shellac above 65% humidity, or when the dew point is within 20F of the air temperature; evaporative cooling can drop the surface up to about 20F.
• Blush in water-based topcoats. General Finishes says rushing dry time can trap moisture between layers and cloud the film. Fix: lengthen intervals; sand back if it persists.
• Pinholes and bubbles (outgassing). Popular Woodworking attributes them to air escaping open pores in woods such as oak and mahogany. Avoid with filled pores and a not-too-wet first coat; brushed varnish bubbles more in hot weather. See Troubleshooting finish defects.
• Soft, tacky film weeks later. Usually coats too thick, too cold, or an oily wood; see Degreasing oily woods.

Safety and environment

Keep cure space ventilated. Oil-finish rags can self-heat; Osmo flags them as a spontaneous-combustion hazard; see Oily rag safety. Do not work below the product's stated minimum temperature (Osmo gives +5 to 35C).

Related

• Wipe-on application, Brush application: where the recoat intervals apply.
• Build coats and sanding between: sanding once a coat is dry.
• Thinning and viscosity: thick coats are the commonest cause of slow cure.
• Tabletop finish schedules: these times put into day-by-day plans.
• Troubleshooting finish defects: blush, bubbles, tackiness.
• Oily rag safety: rag handling and disposal.
• Wipe-on polyurethane, Water-based polyurethane, Hardwax oil: the finishes tabulated.