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What kind of finishes were available?
Nowadays, there is a bewildering range of different finishes it is possible to use, but the 'traditional' finishes were much more limited. Italian mandolins tend to have been finished with shellac, but the tops left bare. Hence the tops of Neapolitan bowls are often very dirty and discoloured, but damage to the shiny bowl finish is easier to repair. German instruments are much more likely to be finished with some form of (highly carcinogenic) nitro-cellulose lacquer, (highly carcinogenic) or more recently, some kind of polyurethane finish, often looking very deep and shiny. This form of finish is quite hard. Other finishes include a variety of oil varnishes, which tend to be more mat or silky in appearance, though not as durable.This is a complex area, made more so by the frankly misleading claims and advice that is often found on the outside of wood-finish containers. Below I have tried to give a summary of sufficient detail to enable the enthusiast to make some sensible decisions about the 'finishing' of a restoration project, without too many disappointments, and sort out some of the mis-labelling and exaggerated claims made on finish containers.
Types of Finish
| Broadly all wood finishes fall into 2
categories; 'film' (forms a film on the surface) and 'penetrating'
finishes (actually penetrates the grain). Within these two
categories the finishes 'cure' in 3 distinct ways, each with
different solvents and thinners. This information is critical to
understanding how finishes behave; which are compatible and which are
not; and with which
types of stain. Its usually easy to tell which type of curing you
are dealing with from the solvent, thinner and clean-up information on
the container, because its often not evident at all from the brand name.
Evaporative finishes: these work when the solvent carrying the finish evaporates leaving a resinous film. The solvents are alcohol, acetone and lacquer thinner. Wax solvent is mineral spirit or turpentine. (shellac, lacquer, pigment stains with lacquer binding, wax)
Reactive finishes: these change chemically when they cure. As the the thinner evaporates, the molecules bond, and cannot be broken down again by re-applying thinner. This is called 'polymerisation'. (This obviously has implications for bonding finishes together when repairing damage.) There two types;
Thinners are mineral spirits or naphtha (often listed as petroleum distillate). (Linseed oil, tung oiloil/varnish blend, wiping varnish, varnish, polyurethane, pigment stains with oil or varnish binder.)
Coalescing Finishes: typically water-based, are a combination of evaporative and reactive finishes. They are essentially emulsions with slow-evaporating thinners. Solvent is glycol ether and the thinner water. (water-based finishes, pigment with water binder, etc)
Which finish is easiest for restoration?
In answering this question I am making an assumption. It is that, the average person wishing to restore an old mandolin, is not likely to be doing many, not likely to have any luthier-specific tools or skills, nor relevant expertise or dedicated premises. On the basis of this assuption, I have ruled out spraying (as requiring all the afore-mentioned) and certain more complex and hazardous options, many of which are really only appropriate for furniture finishing. I have restricted myself to those approaches that could be carried out in a shed or garage, with a minimum of equipment. The final choice rests with you, and depends on your preferences, circumstances and the instrument you wish to bring back to life.
I tend to use oil varnish when refinishing old German instruments, as it is manageable in a small workshop. I use Tru-oil, which is a modern wiping varnish that does not build up a thick layer, and therefore allows maximum vibration of the wood, whilst being fairly hard, durable, and easy to apply.
When refinishing Italian instruments, I use French polish (shellac) which again needs no specialised equipment.
Finally, a more modern and ecologically sound approach, is to try water-based varnishes, which have only really been available for the last 10-15 years.
N.B. If you are repairing a finish rather than replacing one, check the finish repair page.