The finish of a piece of turning is one of the most important things about it. But what constitutes a 'good finish'? That depends partly on what you are making and who it is for. Is it an everyday practical item or a competition entry? Are you making it for yourself, family, to sell, or to impress other turners? Should it be gloss, satin or matte? I think that the smaller the item, the higher the shine should be, but this is a matter of taste. The higher the gloss you want to achieve, the more care must be taken in the finishing.
A good finish starts with the wood itself. Hard, fine grained timber can be finished to a high gloss, but soft, spalted wood will not finish well without heavy sanding and sealing and probably wont look good even then.
A well finished piece must look good and feel good. So before the polish is applied, the item must be crisply turned but without sharp edges, it will have no torn grain, no sunken patches, no unintended ridges or grooves or dents, and no prominent scratches.
Flat surfaces are particularly prone to showing concentric ridges and hollows where the turning/sanding has failed to leave a level surface. It is hard to make the bottom of a platter perfectly flat, and a glossy surface shows reflections that highlight the rings. This is a reason to make them slightly dished in shape if a gloss finish is planned.
Some pieces can be considered complete when the turning is done, without any applied finish. But most will get further treatment. The next step is sanding. Rotary sanders leave less prominent scratches than hand sanding, but they don't get into corners very well. Use fresh abrasive, renewing it when the dust cloud diminishes. Work through all the grits, keeping the abrasive moving. The piece must be thoroughly sanded, but not oversanded, which can damage detailing and will leave sunken patches where softer material is removed faster. Now inspect carefully with a good bright light and your most powerful spectacles. Make sure the whole surface is uniform and scratch-free, including in any corners and recesses, and that the shape is as good as you can get it. Sand to at least 320 grit. You can go to 400-600 or even higher, but this is not really necessary if the piece is to be buffed later. Hard close grained woods that take a shine will show scratches more and need higher grits. Remember that the finish will not improve a poorly prepared surface.
If you are going to use a water-based finish, it can be beneficial to raise the grain at this point. Wet the surface briefly and allow to dry thoroughly. There is no need to soak the item. When dry, sand again with the highest grit you used previously, if possible in both directions of rotation.
Sanding sealer before final sanding may help get a good surface on softer woods.
Blow off the dust, then the bowl is ready for the finish itself. There are lots of options to choose from, including the following:
- shellac - this gives a nice finish with little colour change in the wood. It takes lots of coats to build up a film, it is not very durable, and is prone to runs. Shellac deteriorates in solution, and sometimes commercial mixes are not dated. You can test them by putting a drop on the lathe bed. If it dries hard, it's good. If it stays sticky or toffee-like, throw it out.
- friction polish - this is quick and easy on small items when you get the knack of applying it, but not very durable
- thinned polyurethane looks good, is durable and tough, but many people object to synthetic finishes.
- mineral oil and some vegetable oils such as olive oil are non-drying. They look good when fresh, but will not give a lasting gloss. They can trap dirt. The oil will wash off, but is easily renewed. But don't use vegetable oils unless they will be washed off and replaced frequently, as some go rancid. The oil can be used as a lubricant when sanding, allowing the mud to fill the pores. Soft wax can be melted over the oil as the piece spins, then burnished off, leaving a dry surface.
- danish oil, tung oil, boiled linseed oil and walnut oil are drying oils. They penetrate and harden as they dry, becoming permanent. Oil finishes bring out the grain nicely, but can darken wood and make pale woods look yellow. They deteriorate in the can with age, and the wiping cloths can catch fire spontaneously. Apply plenty of oil, then wipe off the surplus. The first coat will soak in and leave dry patches. Allow to harden for a day, then repeat. Depending on the wood, it will need at least two coats, usually three or more to fill the pores. When the oil stops soaking in, that is enough. Allow plenty of time to dry thoroughly, usually at least two to three days. Longer is better. Rub down to remove any roughness, leaving a low sheen. For a higher shine, use a buffing wheel.
- wax is quick and easy to apply, but not lasting if applied to bare wood. It shows watermarks, but is fairly easy to refresh. It can be applied over any other finish. Microcrystalline wax is probably the most durable and moisture resistant. Beeswax is soft, traps dirt, is not moisture repellant, and it shows fingermarks. But it is a popular choice because of its naturalness and scent. Soft paraffin wax is similar to beeswax. Carnauba wax applied with a buffing wheel is hard and more durable, but still shows water marks.
Any finish top coat may need some form of buffing or mild abrasion to remove dust specks and blend areas together before waxing. The easiest method is often to use a buffing wheel, with or without cutting compound.
Buffing with a wheel is the final step. It gives a very smooth, tactile finish that can be taken to a high gloss if the preparation is good.
My favourite finish is Liberon Finishing oil, buffed to a moderate gloss level, with a top coat of microcrystalline wax applied with a wheel. It is easy, durable, food safe and moisture resistant. With care, this finish lasts indefinitely.