Thinning and viscosity

Overview

Thinning lowers a finish's viscosity so it atomizes, levels or penetrates. It is not free: each thinned coat lays down less solid, so you need more coats. The data sheet is the first authority. The Canadian Woodworking guide says oils, oil/varnish blends and hardwax oils typically need no thinning, since they are meant to go on thin by rag or brush, and that water-based finishes thin with water or a retarder.

When to use

• First coat on bare wood, or a homemade wiping varnish; see Wipe-on application.
• Brushing flow; see Brush application.
• Spraying: only as much as the gun needs; see Spraying with HVLP.
• Not when the maker says not to: some water-based varnishes don't tolerate thinning (Woodcraft), and General Finishes' Arm-R-Seal data sheet says spraying is not recommended.

Materials and tools

• Matching thinner: mineral spirits for oil-based varnish and polyurethane; denatured alcohol for shellac; lacquer thinner for lacquers (for spray, the maker's own thinner); clean water or a maker's retarder for water-based (Canadian Woodworking). Start Woodworking Now, a smaller blog, adds that using the wrong class of thinner, particularly mineral spirits in a water-based finish, can curdle it.
• Naphtha is more volatile than mineral spirits, so it shortens drying and is more flammable; turpentine is slower, pungent and costlier (Canadian Woodworking).
• A #4 Ford viscosity cup (Fuji includes one) and a timer.

Steps

1. Check the label or data sheet for the thinner and limit.
2. Oil varnish or polyurethane: 3 to 4 parts varnish to 1 part mineral spirits for brushing, though the ratio isn't crucial; about 50% for wiping (Canadian Woodworking). Woodcraft also uses about 50% for the first soak-in coat, and up to 100% for very thick varnishes. Brushing first coats: 10 to 50% (Woodcraft).
3. Bubbles in brushed varnish: add 5 to 10% mineral spirits (Flexner).
4. Shellac: thin with denatured alcohol; the exact ratio isn't critical (Canadian Woodworking). Methanol is best avoided, as highly toxic and more flammable.
5. Lacquer: use thinner meant for thinning, not cleanup, or the finish can come out of solution as white fuzz, "cotton blush" (Flexner).
6. Water-based: add water or extender gradually; General Finishes' extender contains propylene glycol per Canadian Woodworking.
7. Spraying: time the finish through the cup. Fuji's rough guide for its guns: 8 to 17 s suits a 1.0 mm cap, 17 to 25 s a 1.3 mm, 25 to 37 s a 1.5 mm. The aim is to thin as little as possible. Record the time that works so batches match.
8. Mix by volume in a jar, stir, never shake, and test on scrap.

Common problems

• More coats than expected: over-thinned. Canadian Woodworking says a few extra for thinned brush-on, and a dozen or more for wiping varnish.
• Curdled or clumpy finish: wrong thinner (see above). Strain and start again.
• Dry spray or blush: too fast a thinner; see retarder in Curing conditions and time-to-use and Troubleshooting finish defects.
• Orange peel: sprayed too thick; thin more (Flexner).

Safety and environment

Thinners are flammable and toxic. Canadian Woodworking recommends a half- or full-face respirator with organic vapour cartridges, splash goggles and nitrile or neoprene gloves, and spreading rags in a single layer to dry. See Oily rag safety.

Related

• Spraying with HVLP, Airless and air-assisted airless spraying: viscosity for guns.
• Shellac, Nitrocellulose lacquer, Oil-based polyurethane, Water-based polyurethane: thinners per finish.
• Build coats and sanding between: coat count.