What is Varnish
Varnish is a general name, like "paint", for a transparent, protective finish or film mostly used on wood. Traditional Varnishes contained a resin, a drying oil, and a solvent.
The finish is usually shiny but can be modified by adding flatting agents or by rubbing the hardened varnish with abrasives. Varnish is transparent, but often has a yellowish tone. Traditional varnishes tend to get a golden hue over time, this is much prized by boat owners. Varnish usually has no pigment added although some varnishes will have a small amount of stain to make the colour more golden. When wood is to be finished so that it has a different colour or much deeper shade, a stain is used first. The varnish is then applied to provide protection and waterproofing.
Epoxy is sensitive to Ultra Violet degradation and needs to be protected from the sun. That's why epoxy is varnished or painted.
How does Varnish Harden or Cure?
Varnish either hardens immediately as the solvent evaporates and creates a film, or hardens gradually (or cures) over a period of time. The chemical process can be complex and involves oxidation and polymerization.
Finishes such as Shellac, Resin varnishes and Lacquers dry and harden immediately to a hard finish.
Acrylic and some water based finishes evaporate the water (and other included solvents) first and feel dry to the touch but gradually harden and cure over a period of time. This is what happens when a finish feels dry but can easily be damaged until it has cured.
Oil based, polyurethane, and epoxy varnishes very gradually harden and go through a series of stages of drying and hardening.
The temperature and humidity of the air has a huge influence on how fast a varnish dries and cures. As well different oils and resins dry and cure at different rates. The ratio of ingredients also has an impact on the dry-cure rate. The warmer and dryer the temperature, the faster the drying and curing.
What Goes Into a Varnish?
Varnish is not a fixed formula and through the ages various components have been added to improve certain characteristics of a varnish such as hardness, speed of drying and cure, elasticity, colour or UV protection.
Modern varnishes now include synthetic additives and components that were not available in the past to improve the qualities of varnishes. There are additive that speed up drying, inprove flow, reduce skinning on un-used portions, improve UV resistance, modify the finished surface to be less shiny or slippery.
Typically a varnish will have a drying oil, a resin that adds strength and body and a solvent to thin the mixture and make it easier to apply. Of course a varnish can contain a mixture of several different oils, resins and solvents in a single preparation.
Drying oils are oils that once exposed to the air will eventually dry, cure, and harden creating a solid film.
There are a great number of oils which can, and have been used, to make varnish such as linseed oil, tung oil, fish oils, and walnut oil. Often oils used in paints and and varnish contain a high level of polyunsaturated fatty acids.
In the past, the preparation of the oils was often extensive and one can still find instructions for preparing linseed oil by heating it and exposing it to the sun causing it to partially oxidize and polymerase. This shortens the time the oil takes to dry. It is not unusual to find metallic salts added to oils intended for varnishes, lead was a common additive in the past as was manganese. These salts hasten drying acting as catalyst.
The proportion of oil in the varnish changes the flexibility of the finish considerably. Old fashioned books talk of long oil varnish, short and medium were also available. The long oil varnishes are extensively used as spar varnishes and in finishes used where weather resistance and flexibility is essential. Short oil varnishes is much more brittle and hard and can be brought to a beautiful high gloss. These are more commonly used on furniture.
Various oils bring different qualities, fish oils made more heat resistant varnishes while tung oil increased water resistance. Increasing the wax content of a varnish can reduce gloss and was used as a flatting agent. This worked but made the varnish difficult to recoat or paint over. More modern alternatives for flatting the finish have been developed.
Here is a Gutenberg book in html format on paint and varnish technology. Published in 1911, the first chapter is on oils and gives an idea of the traditional processing of oils for paints and varnishes.
Resin is an ill defined term and has been used for a wide range of substances. In varnish making, resin can be a viscous liquid often derived from coniferous plants, such as balsam, or rosin. It can also be a fossilized resin such as amber or copal or Kauri gum (fossilized Kauri tree gum found in New Zeland and similar to amber.) Damar is a gum harvested from Dipterocarpaceae trees or as a fossilized form. Sandarac, Mastic, Anime, and Elemi are also plant derived resins that are or have been used historically in varnish.
Some resins are quite dark and impart significant colour to the finished varnish, a characteristic that is usually undesirable. Resins impart more or less hardness to varnish and paints, The greater the proportion of resin to oil, the greater the hardness and brittleness of the varnish.
In the finishing industry many substances get called resin.
Lac is a resin secreted by an insect and disolved in alcohol creating shellac.
Acrylic resin is a general term for a range of plastics made by polymerizing a monomer using heat and an initiator. The resin produced from methyl methacrylate monomer (MMA) is polymethyl methacrylate (PMMA) or Acrylic resin. MMA is a transparent and colorless fluid substance. One of the main characteristic features of PMMA is its high transparency. It is useful where the finish is to remain as transparent as possible.
Phenolic resins are also used in high end spar varnish such as Epiphanes.
Modern finishes usually use Alkyd resins derived from polyester modified with fatty acids and other components, along with naturally derived resins.
Traditionally, natural (organic) turpentine was used to thin varnishes to usable consistancy, but in modern varnish it is more common to use several mineral based solvents such as white spirit or "paint thinner", also known as "mineral spirit". These are obtained from the distillation of crude oil
Turpentine is a liquid resulting from the distillation of mainly pine trees. The original tree was called a terebinth tree thus the name turpentine.
The amount of solvent helps modify the thickness of the varnish and improve the ease of application. In quality varnish a high level of solids are expected and overly thinned varnishes are frowned upon. If the varnish is to be sprayed on it needs to be thinned.
Because of environmental regulations regarding the proportion of solvent in finishes, manufacturers have been struggling with trying to reduce the proportion of organic solvents in their varnishes. Valiant efforts have also been made to develop water based finishes. The jury is still out as to the merit of these. Minwax makes a water based Spar Varnish
There are many kind of varnishes being made
Different final use require different formulations of varnish. In the past and still today, great secrecy and secret formulas surround the making of varnishes.
Violin or Instrument Varnish
Because the tone of a musical instrument can be greatly influenced by the finish applied, a great deal of experimentation has been done on the subject. Famous instrument makers had their closely guarded formulas and procedures. Expert finishers are worth their weight in gold. Many exotic combinations have been tried but often the finish was a simple traditional varnish.
Keith Hill is an instrument maker and offers his recipe for instrument varnish here.
Leroy Douglas Violins also has a large interesting section on violin varnish and finish. None of it is straight forward but it is an interesting read.
Shellac is a resin varnish that is alcohol soluble. It is mostly used as a sealing primer these days because of its sensitivity to alcohol. Shellac resin is a brittle secretion of the female lac insect, Kerria lacca, found in Thailand and gathered from the bark of the trees. When used as a primer it is generally dewaxed. It is useful under polyurethanes, alkyds, oils, and acrylics. Shellac comes in various concentrations, the "cut" is measured in pounds per gallon. 3 pound cut is a common concentration.
Shellac comes in various shades of orange, golden or almost clear.
Shellac has the virtue of sealing in and blocking surface stains and to some extent sap. It is commonly used as an ingredient in pigmented blocking primers such as Zinsser. It is still widely used as a sealer/primer coat under other finishes and varnishes.
Alkyds are a group of synthetic polyester resins modified by the addition of fatty acids to give them the ability to form flexible coatings. They are derived from various alcohols and acids. They are the more common resin used for modern oil based varnishes and paints.
Alkyds are combined with various drying oils to make varnishes and paints. Because the fatty acids are plant derived Alkyds have not gone up in price as much as petroleum based products.
Various additives, dryers and modifiers can be added to improve and modify desired qualities. Thus alkyd varnish can have a wide range of characteristics for many applications. In particular it can be made more or less flexible, or more or less brittle.
Marine or Spar varnish has a long history of use on boat parts where the varnish's main job was to protect wood exposed to often harsh marine conditions. Spars and masts bend and spar varnish has to be flexible to accommodate this and not crack or flake of. Primary requirement of spar varnish is flexibility and impermeability combined with UV resistance. Modified tung oil and phenolic resins are often used.
The virtue of spar varnish lies in its elasticity and water resistance and much less on its appearance. This spar varnish is often not as shiny as other varnishes. It remains too soft for this.
Typically spar varnishes require many coats and rely on this layering to provide enough UV protection.
Spar varnish is sometimes referred to as long oil varnish. This means that spar varnish has a much higher concentration of drying oils to resin ratio than varnish that are going to be used for furniture which are referred to as short oil varnishes.
The higher the proportion of oil to resin, the softer and more flexible the varnish. Less oil in the mix, results in a harder more brittle and shinier varnish.
Spar varnish has a reputation for being tough and some manufacturers have named their varnish "spar" to capitalize on this belief. Spar varnish is not necessarily the best varnish for inside finishing projects as it remains softer than ideal.
Many manufacturers produce a wide range of spar varnish with various claims made of them. Whatever the characteristics claimed, a spar varnish will be flexible, softer than other finishes and hopefully will have UV protection. More expensive spar varnishes will have a higher concentration of solids to solvent so fewer coats will be needed.
Traditional spar varnishes are less available than they were because of environmental regulations regarding solvents. Because varnishes in general have a high ratio of solvents they are increasingly banned. Epoxy Products has a page on traditional spar varnish.
An oil finish is not strictly a varnish but it has been used for a long time. Oil finishes are not very waterproof, nor do they provide much abrasion protection. In old ships various mixtures of tar and oil were used to coat wood and help provide improved sun and weather resistance.
Typically linseed or tung oil mixed with some solvent to improve wood penetration, is used. Iain Oughtred has experimented with oil finishes and it is commonly used in Scandinavian boats such as Faerings. Although not waterproof an oil finish does not trap moisture either.
After a while the oil dries and polymerizes or cures. Sometimes it can take a quite long time for this to happen. Often oil finishes are also waxed. Oil finishes have a nice glowing appearance.
Polyurethane varnishes are typically hard, resistant, and durable. Sometimes offered as 2 part finishes, Polyurethane is used for hardwood floors (do you remember the roller skater on the living room floor ads?). Polyurethanes are comparable in hardness to alkyds but generally form a tougher film.
Polyurethane may de-laminate if struck or heated, it does not penetrate deeply in the wood and this increases the risk of delamination. A good primer is helpful with polyurethane finishes. Shellac is sometimes used as a primer. In more demanding applications penetrating epoxy is another possible primer. Polyurethane can be applied on top of properly cured oil based finishes. Oil finishes are not as happy on top of polyurethane and might delaminate.
Polyurethane is not UV resistant and if it will be used outside UV resistant modified polyurethane should be used.
Besides its exceptional harness polyurethane offers good chemical resistance to fuels, oil and mild acids and alkali. Another virtue of polyurethane is the clarity of the finish. There is usually little yellowing.
Lacquer is a quick-drying, solvent-based finish applied by spraying. Lacquer is dissolved in lacquer thinner not mineral spirits or alcohol. Acetone is the usual solvent base. Once applied, lacquer can be removed using lacquer thinner. It does not cure or polymerize.
AcrylicAcrylic varnishes are usually water-based varnishes with the low refractive index and high transparency. They resist yellowing. Acrylic varnishes are often used to seal acrylic artwork. It does not penetrate into wood as well as oil based finishes.
Although epoxy finishes are not strictly varnishes they are often used in similar applications. Typically epoxy comes in 2 parts that are combined. The epoxy sets chemically to a hard finish. It is sensitive to UV degradation and any epoxy coating needs to be protected if used on a boat or outside. Several penetrating epoxy varnishes have been made and are widely used in boatmaking applications. The rate of cure of epoxy is dependent on the temperature and the type of hardener used. The warmer the temperature the shorter the pot life of mixed epoxy. Very cold conditions inhibit the setting. Most epoxy resins are similar but there is quite a lot of difference in the hardener part. Hardeners have been developed which can set in quite cold conditions. Other hardeners are much slower and allow users to work in quite warm conditions or allow open time to be extended.
Penetrating epoxies have been offered for impregnating rotten wood in boats and restoring some strength. Results have been mixed. It is more successful as a finish on new wood.
This page is a result of research I'm doing on varnish. It relies heavily on information found in Wikipedia and on information provided by manufacturers. I'm not personally an expert. Mostly varnish work. The few failures I've had have been the result of improper preparation or contamination of the surface before applying the varnish. In varnishes as in anything else you get what you pay for. A bargain priced varnish might work very well but have a smaller proportion of solids so you need more coats. It ends up costing the same for a good finish.
I don't claim to be an expert in anything. Use safety equipment when working on your boat and wear a lifejacket when boating. Many coatings and paints are toxic or dangerous if used improperly. Many finishes have flammable solvents and components. As they dry, paints and varnishes often give off heat so any rag should be disposed of carefully and not left where the accumulation of heat can occur. Before using any product READ the label and take the warnings seriously.email me if you find mistakes, I'll fix them and we'll all benefit: Christine