Soap finish and lye (Scandinavian)

Overview

A soap finish is a Scandinavian tradition: soap is worked into bare wood, leaving a thin, pale, soft-to-the-touch surface that is cleaned and renewed with more soap. Often it follows a lye treatment that whitens the wood. Reed Hansuld's Fine Woodworking article describes a quick version with pure soap flakes whisked with hot water to a stiff froth. WOCA calls the finish soft to the touch and light, best suited to light-toned softwoods such as pine and Douglas fir and to oak. It is a look and a maintenance routine, not a hard-wearing finish.

Pros

• Pale, matte, non-amber look; Fine Woodworking says it adds no yellow and has zero VOC with no PPE needed.
• Very simple, cheap and quick; shop-made versions need only soap flakes, Castile soap and hot water.
• Easy to repair and renew; spot-treat stains with very hot water (Hansuld).
• Pleasant, dry feel.

Cons

• Light protection. Hansuld says it "won't take abuse like varnish or epoxy" and renews about once a year.
• No UV protection, so the wood darkens with light.
• Poor on woods that darken or have strong colour such as walnut, cherry and rosewood (Hansuld's advice); he would hesitate to use it on veneer.
• Water spotting and stains are likelier than with film finishes.
• Tannin-rich woods can react with tap water; Woodcraft recommends distilled water to avoid tannin reactions.

Suitability

Suits pale open-grained or softwood pieces: White oak, European ash, Hard maple, European beech, Eastern white pine. Furniture, shelves, table tops with light use. Avoid on Black walnut, Black cherry and East Indian rosewood, where the pale look is lost and the colour may suffer. Not for kitchens, bathrooms or worktops that see a lot of water.

Coats and application

Lye (optional): Biofa's brochure says its lye whitens and brightens the wood and reduces yellowing, with hardwood lye 2093 and softwood lye 2094 for different woods; WOCA describes its Wood Lye as a pretreatment that neutralises yellow and red tones. Lye is caustic; use gloves and eye protection, follow the label, and neutralise as the maker directs (Allbäck says a lye-treated floor must be neutralised with acetic acid and water before linseed paint or wax).

Soap, Hansuld's recipe: whisk a handful of pure soap flakes with about 1/2 cup of hot water to a stiff froth, rub on with a cotton rag, then buff with a fresh rag. After about an hour, lightly sand (220, working up to 600 or finer) for a smooth surface. Woodcraft's variant uses Castile soap with boiling water. Sand to 180 to 220 beforehand (Sanding schedule; Raising the grain helps on pine).

Compatibility

Soap finish is a final treatment on bare wood or lye-treated wood. Do not put varnish, lacquer or other finishes over soap. A wax can be added for more protection (Paste wax), though that changes the character. See Hardwax oil for a tougher natural look.

Care and repair

Clean with a soap solution and renew the finish, about yearly (Hansuld). Biofa says the soap system is renewed along with cleaning. Spot-treat stains with very hot water. Light scuffs can be re-soaped without sanding.

Safety and environment

Lye (sodium or potassium hydroxide types) is caustic and can burn skin and eyes; wear gloves and goggles and follow the product label and SDS. Soap flakes are low hazard but can irritate eyes and skin. Unlike oil finishes, soap does not carry the oily-rag combustion hazard, but rags soaked with oil after soap treatment do; see Oily rag safety. No food-contact claim is made for soap finishes here; see Food-safe finishes.

Related

• Hardwax oil: tougher natural look.
• Paste wax: topcoat for soap-finished wood.
• Sanding schedule: preparation.
• Water-soluble dye stain: other colour control.
• Oil and wax blends: oil-based natural alternative.