Basic Finishing Guide archive

Tuesday 15 December 2009

Nick Arnull presents a basic guide to finishing

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This article is aimed at those new to woodturning but I feel sure there is something here for turners of all levels. Today, we are fortunate to have many products available to us, some of which have become widely available and others only by region. Finishing is a huge subject for woodturning and here I will only touch upon some basic types to get you started.

Often the beginner will find it difficult to achieve a satisfactory finish and will use one finish that does it all. This is limiting, however, and does not take into account the different ways that wood needs to be treated depending upon its final use. Is the piece purely decorative? For food contact? Or, will it get a great deal of handling? Are you unsure what you want to do with the piece, and do you want to show how attractive the wood grain is in the piece?

When viewing work it is often the type of finish or standard of finish that lets the piece down.

The biggest mistake made by many is to apply too much finish and end up with streaky or patchy finishes along with poorly executed sanding. This article aims to address these mistakes.

Finishing products

Sealers

This group includes cellulose, acrylic and shellac varieties. The group also includes pre-catalysed melamine lacquer, which is a water resistant sealer. These are used to seal the wood before applying a finish, but they can also be used as a final finish in their own right. Sanding sealers are diluted 50% using the appropriate thinning agent to allow application over larger areas whilst staying wet across the entire piece, also allowing it to flood into details. If undiluted it is often impossible to get an even coverage with the product straight from the tin.

Waxes

These come in different types, such as soft paste waxes, hard stick varieties and can be coloured. These are usually best applied over a sealer, but some new types can be applied to bare wood.

Durable hard-wearing finishes

Lacquers and oils fit this section well. Some can penetrate the timber; others form a surface finish and come in gloss, satin or matt sheen. Oil is deemed to be one of the most durable but not necessarily the best at resisting finger marks and dirt contamination.

Decorative finish

A type of finish that will modify or change the appearance of the wood. On some occasions it may totally cover it to a point where it no longer looks like wood. This complex group includes coloured stains and waxes that are applied to bare wood along with chemicals, such as bleach.

Food safe

If the piece comes into contact with food then it has to be food safe and that means it must comply with current government standards (if these are applicable). Alternatively, you could use pure beeswax, vegetable/mineral oil or liquid paraffin, all of which are applicable, plus many other manufacturer specific types of finish.

Abrasives

These should be treated as another cutting tool and you really do get what you pay for where these are concerned. Good quality abrasives are not cheap; they need to be cloth backed and flexible and must not crack when folded. These are usually made of aluminium oxide and are heat resistant and waterproof to some degree. The grades I use are 80, 120, 180, 240, 320, 400 grit. I invariably start sanding at 80 grit because my turning tools are sharpened on a dry grinder that is fitted with an 80 grit white aloxite grinding wheel, which produces 80 scratches per inch along the cutting edge. 80 grit abrasive will remove any lumps and bumps from the workpiece very quickly. When working as an apprentice stonemason polishing granite I recall that if you missed one grade you had to start all over again - there are no short cuts to the perfect finish.

Polishing cloths and papers

Some say cloths, some say papers, but I use both depending upon the job in hand. Mutton cloth is used for applying sanding sealer and polishing waxes, whereas paper is used for friction polish and oils. But remember that all paper cloths have an abrasive quality of their own, so it may take time to find the right one for you. Start with a good quality kitchen paper towel, but ensure to NEVER wrap any polishing cloth or paper around your fingers. Also, never allow loose ends to trail around rotating machinery or work.

Wire wool and Nyweb/Webrax

0000 wire wool is used for cutting back finishes and sealers. Nyweb and Webrax are artificial forms of wire wool that do not degrade as readily as wire wool and do not leave debris behind as they are used.

My basic finishing technique

The Basic method for finishing is to sand, seal and then apply the finish. When you are using oil, however, there is some change in this method.

Sanding the exterior

1.

Reduce the lathe speed by half and using a piece of abrasive folded in two to sand the outside of your bowl. Remember to keep the abrasive moving, never allow it to stop in one place or there will be scratches in your final finish. Work progressively through the grades from 80-400

2.

With the lathe stationary use a 25mm (1in) paint brush and apply a generous coat of sanding sealer to the bowl exterior, working rapidly to get the entire surface wet. Do this quickly or one part will dry before another resulting in a patchy finish

3.

With the lathe still turned off remove the excess with mutton cloth. When it is dry turn on the lathe and burnish with the cloth

4.

Using Nyweb cut back the surface to remove the raised grain and excess product. Repeat the process for a second coat

5.

Sparingly apply a coat of paste wax and allow to dry. Be patient as drying times vary throughout the year

6.

Turn on the lathe and with a clean cloth burnish the surface of the bowl. Move to a clean spot on the cloth and polish the bowl

Sanding the interior

1.

Follow the same method for sanding the interior of your bowls, but this time sand the bowl in the bottom left quarter between the 6 and 9 o'clock positions. This is considered the safe area to sand and is where you are unlikely to have the hand shoot around the top of the bowl. Remember to keep the abrasive moving in to minimise radial scratches on the workpiece

2.

Alternatively, power sand the interior. Notice the angle at which the head is presented to allow only the lower edge to contact the rotating bowl

3.

Try fitting a larger sanding pad than you might normally use. The larger the pad, the more surface contact you have - a better match to the radius. This larger surface contact has the advantage of being able to even out any surface undulations better than a small pad can

4.

You could also use an inertia/ passive shear sander to remove any radial sanding marks that may still be visible on the surface

Method for applying an oiled finish

Applying oil takes time but it does give the wood a much greater depth and richness. When sanding with oil the amount of fine dust generated in the workshop is much less, it will also grain fill leaving the wood with a much softer and warmer feel. This is ideal when making items that come into contact with food. Sand as you would normally, ensuring no damage is left on the surface of the bowl. When finishing with oil it is a good idea to cover the lathe bed. Here, I decided to use Danish oil

1.

Apply a liberal coat of oil to your bowl using a 25mm (1in) paint brush and leave to sit and drip for 10 minutes. During this time the oil will be absorbed into the surface of the timber

2.

Replenish the oil and using old 400 grit abrasive turn on the lathe at its slowest setting and proceed to abrade the bowl as the sanding progresses. If the abrasive feels like it is grabbing, add a little more oil and continue

3.

Continue to sand the bowl until you get a slurry appearing on the surface of the timber. At this point continue for a little longer

4.

Wipe clean and dry, then run the lathe and burnish with paper. Now set the piece to one side in a dust free area and allow to dry for 24 hours. The following day add another coat of oil by brushing on and leaving to sit for a while, then wipe dry. Continue until you get the desired finish (I find that two or three coats are usually adequate). Once the oil is fully dry it may well require a polish using a polishing mop loaded with a cutting compound; wax sometimes is enough to bring the piece to a very good shine

Friction polish

This product is often misused by beginners and should only be used on small items, such as light/cord pulls and bottle stoppers. Most manufacturers state that no sealer is required but I find this is not the case - you can achieve a far superior finish when using one.

1.

Sand the piece as usual. Sanding on top allows you to clearly see what you are doing

2.

Use a small brush to apply your chosen sanding sealer and place a small piece of mutton cloth to the rear of the piece with the lathe running - this will catch the excess and burnish it simultaneously

3.

Continue to burnish. If you have too much product on the piece some of it will be removed during this process

4.

Using Nyweb cut back the surface to remove any further residue

5.

With your brush dipped into your friction polish apply as you did the sealer, but this time use a paper towel folded in half

6.

Give the piece a final polish using a clean area of the paper


Woodworkers Institute

Tagged In:

beginner , Nick Arnull , Finishing

Glossary Rollover a term to view its definition

Lathe , Power Sanding , Danish Oil , Steel Wool , Finishing

Handy Hints

1. Reduce lathe speed when sanding to decrease heat being generated. This will extend the life of your abrasive
2. Store wire wool in an air-tight container to keep it clean and safe
3. Do not keep using sandpaper that is no longer doing the job - it is false economy
4. Keep rags in a clean place
5. Decant your finishes into smaller jars to allow application to be made by brush