Wood Treatment

Shellac functions as a versatile wood treatment and coating, and acts as a binding agent in paint and rust treatment. It is commonly used as a primer, basecoat or undercoat prior to painting, a knot and stain sealer and odour blocker, as well as a high-gloss wood varnish, being a key component of the French Polishing process. Shellac is also resistant to ultra-violet light, so does not darken with age, and is both quick drying and cures at relatively low temperatures. Damaged Shellac can simply be touched-up with a fresh coat of Shellac because the new one amalgamates with the existing coat. Brushes can be cleaned using methylated spirits.

As a natural resin, Shellac is compatible with many other finishes. After dissolving in a liquid solution, coloured Shellac can be used as a brush-on stain, while clear or blond Shellac can be applied without affecting the colour of the wood. Shellac applied as a protective topcoat finish provides a barrier against water vapour penetration, protecting wood against moisture damage.

Shellac is a key component of the French polish method of finishing wooden furniture, particularly for antiques. Luthiers use Shellac to French polish stringed instruments, including guitars, violins and pianos, and to help bind the wood fibres. Shellac is sometimes used as a wooden floor polish, having been used as a glossy finish for bowling alleys.

The concentration of Shellac used for wood treatment is commonly known as the "pound cut", which refers to the number of lbs of Shellac dissolved in one US gallon (0.83 Imperial gallons or 3.79 litres) of denatured alcohol (Methylated Spirit). The choice of Shellac grade depends on the colour of the wood and the desired finish, with grades ranging from transparent (bleached) to light yellow (super-blonde) to dark orange/brown (garnet). When applying the solution with a cloth pad, the first coat (washcoat) typically requires a 1-2 lb cut, followed by the second coat (sealer coat) at 1.5-2.0 lb cut for close-grain wood or 2.0-2.5 lb cut for open-grain wood, before the final coat (topcoat) at 1.5-2.0 lb cut. When brushing-on or spraying the solution, the washcoat is 0.5-0.75 lb cut (25 lb spray pressure), the sealer coat is 2 lb cut (30 lb spray pressure), and the topcoat is 2-3 lb cut (30 lb spray pressure). Spraying a 4 lb cut requires a spray pressure of 40-45 lbs. The spray should be applied in a fan pattern approximately eight inches from the surface of the wood, but should never be attempted when the relative humidity is over 70%. French polishing typically requires a 0.75 to 1.5 lb cut, with a small quantity of butyl alcohol added to prevent rapid evaporation of the solvent. The applying pad usually comprises some cotton wadding covered with a piece of linen sheeting. Three or more coats are applied, with light sanding with number 0 paper taking place between each layer after they have fully dried overnight. A few drops of linseed oil are used to lubricate the pad to prevent dragging as the layers build up. When using Shellac as a wood sealer or undercoat a 2 lb cut is used, while a 3-4 lb cut is used when sealing plaster.

The table below converts the pound cut into metric standards:

Pound Cut Grams of Shellac per litre
0.50 60
0.75 90
1.00 120
1.50 180
2.00 240
2.50 300
3.00 359
3.50 419
4.00 479

 

A.F. Suter is a leading UK supplier, retailer, wholesaler, distributor, importer and exporter of high quality Shellac, Waxes, Natural Gums, Resins, Menthol Crystals and Zein.

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