Surface preparation and finishing: HSC Industrial Technology Timber Products and Furniture Technologies

What this dot point is asking

The finish is the most visible part of a finished project and a large part of the practical mark. NESA expects you to describe how a timber surface is prepared for finishing, the function and properties of the main finish types, how they are applied, and how to choose a finish that suits the timber, the use and the appearance you want. Because finishing is so often rushed at the end, understanding it well protects both your project mark and the durability of the work.

Why we finish timber

A finish does several jobs at once. It protects the timber from moisture, dirt, wear and ultraviolet light; it enhances appearance by deepening colour and revealing grain or by adding colour; and it makes surfaces easier to clean and more pleasant to touch. Bare timber stains, swells with moisture and wears, so almost every piece of furniture is finished.

Surface preparation

A finish reveals every flaw beneath it, so preparation is half the job. The steps are:

1. Level and smooth: plane or scrape surfaces flat, then sand through progressively finer grits, removing the scratches of each grit before moving up.
2. Fill defects: fill nail holes, gaps and open grain with a matching filler or grain filler so the finished surface is even.
3. Sand with the grain: always sand along the grain, because cross-grain scratches show badly once a finish is applied.
4. Remove dust: vacuum and wipe with a tack cloth so no dust is trapped in the finish.

Test the finish and any stain on an offcut of the same timber first, because species absorb finish differently.

Penetrating finishes

Penetrating finishes soak into the timber rather than forming a thick film:

• Oils such as linseed, tung and Danish oil soak in, give a natural low-sheen look that feels like timber, and are easy to repair by re-coating. They offer less protection against heavy wear and water.
• Waxes add a soft sheen and feel, usually over another finish rather than alone.
• Stains are colourants that penetrate to change the timber's colour without hiding the grain, applied before a clear protective coat.

Surface-forming finishes

Surface-forming finishes build a protective film on top of the timber:

• Varnishes form a tough, durable, often glossy film with good water and wear resistance, suiting tables and high-use surfaces.
• Lacquers dry quickly to a hard, clear film and are usually sprayed, common in commercial furniture production.
• Paints add an opaque coloured film that hides the grain, ideal over MDF and for painted furniture, applied over a primer.
• Shellac is a traditional film finish that seals and gives warmth, often used as a sanding sealer or base coat.

Application and selection

Finishes are applied by brush, cloth or pad for oils and stains, and by brush or spray for film finishes. Work in a clean, dust-free, well-ventilated space, apply thin even coats, and sand lightly between coats of a film finish to build a smooth surface.

Choose the finish by matching it to the job. Use a durable varnish or lacquer for a dining table that meets food, heat and spills; an oil for a piece where natural feel and easy repair matter; paint over MDF where you want colour and a smooth opaque surface; and always consider the sheen, the timber species and the time and equipment you have. Justify the choice in your folio against protection, appearance and use.