Why is my varnish still tacky and not drying?

Varnish cures rather than simply drying, and that reaction depends on temperature, air movement and what it is sitting on. Almost every failure is one of those three, and the difference between slow and never is a matter of days.

What you'll need

• heat and ventilation
• white spirit or the manufacturer's recommended solvent (optional)
• abrasive and scrapers (optional)
• fresh varnish (optional)

Step by step

1. Check the temperature and humidity first.Most varnishes need a minimum of about 10 to 15 degrees and struggle in high humidity. A cold damp garage in November is the classic setting for this problem.
2. Warm and ventilate the space.Gentle warmth and moving air, not a heat gun. Both are needed — warmth alone in a sealed room does very little.
3. Give it time before acting.In poor conditions a coat that would cure in a day can take a week. Wait several days before deciding it has failed.
4. Consider the coat thickness.Oil-based varnish cures from the surface inwards. A thick coat skins over and the material underneath stays soft, sometimes indefinitely.
5. Think about what was underneath.Wax, silicone polish, oily timbers such as teak and rosewood, and residues from cleaning products all inhibit curing. So does an incompatible previous finish.
6. Check the age of the tin.Part-used tins skin over and the remaining varnish can be past its useful life. Old or contaminated stock is a real cause of a non-curing coat.
7. Check compatibility.Water-based varnish over an uncured oil-based stain, or the reverse, frequently stays soft. Use a compatible system and let each layer cure fully.
8. Remove a failed coat properly.White spirit and a cloth for oil-based varnish, or the manufacturer's stated solvent. Work in sections and change cloths often.
9. Prepare the surface again.Once stripped, degrease with methylated spirit or the recommended cleaner, sand back to clean timber, and remove all dust.
10. Reapply thinly in decent conditions.Thin coats, warmth, ventilation and the full recoat interval between them. Three thin coats give a far better and more durable finish than two thick ones.

Tips

• Warmth and airflow together. Warmth alone in a closed room does surprisingly little for curing.
• Wax or silicone polish on the timber is the classic invisible cause. If a piece has been polished for years, degrease before finishing.
• Thin coats. Varnish cures from the outside in, so thickness is the enemy of curing.

Common mistakes

• Applying another coat over a tacky one — The new coat seals the soft one underneath and neither cures. It turns a delay into a stripping job.
• Varnishing in a cold garage in winter — Below the product's minimum temperature the curing reaction effectively stops. The finish stays soft and may never harden even if the room is warmed later.

If it doesn't work

Questions people ask