Oil, wax, shellac or varnish? How to find out what's on the surface of your furniture and what you can refinish it with

"Maybe write an article about mixing different paints, varnishes and wood preservatives, possibly how special they can be. I've seen on multiple occasions people who have forgotten what they or the workmen have been messing with, then bought another type of paint and applied it. Afterwards they thought they could mix anything anyway."This is a message from one of our readers. It's true, we've touched on the subject incompatibility between layersbut it is equally true that I often see misguided recommendations on profile sites that more confuse the advice seeker than help. Then it's time for clearer and more to the point explanations.

Restoration of wooden objects finished with natural materials

The materials with which the wood is covered have an aesthetic and protective role. Even if a very resistant hardwood is used, protection is necessary to keep it from water, stains or dirt accumulated over time. A wide variety of products are used for coating. Whereas in the past only waxes, oils and shellac, With the development of the chemical industry the range has diversified greatly. New varnishes and paints have become increasingly resistant, meeting the requirements of the areas where the finished object is used. We often want to restore pieces of furniture that we have from our parents, grandparents, and that we have memories of. The very old ones are mostly coated with wax, oil, shellac or nitrocellulose varnish. They are less resistant materials and cannot be coated with any kind of varnish or paint.

What works and what doesn't work as a finish over wax, oil, shellac or nitro varnish

Ceara is the final layer over which nothing else can be applied. If you want to repaint a waxed object, you must first remove the wax. This also applies to wax accumulated over time from maintenance products.

Pis o oiled surface only oil or wax can be applied. As old objects we are talking about natural oils, not today's synthetic oils, which are a kind of modified varnish. Oiled surfaces are usually poor in surface finish and often, because of age, the wood has a dry appearance on some portions. Natural oils are compatible with each other, so a wood finished with tung oilfor example, can be re-operated with linseed oil or withDanish Oil. Wax can also cover a waxed finish, helping to protect it.

Surfaces finished with shellac or nitrocellulose varnish behave similarly. Nitrocellulose varnish is one of the first synthetic varnishes to appear and like shellac, is a resin dissolved in solvents. Both can be coated with either shellac or nitrocellulose varnish. Do not apply chemical-curing, solvent-based varnishes over shellac or nitrocellulose varnish. The solvent in the hardening varnish will soften the varnish layer underneath. Thanks to the catalyst, the top layer will harden, trapping the solvent inside. This will continue to dissolve the original varnish, which will eventually come to the surface, breaking up the thin film on top. Craftsmen call this phenomenon the blooming of the varnish.

How to identify the coating material with which the wood has been protected

In order to know what we are applying on top we need to know what is already applied by the piece of wood, both for new and old pieces. Testing is done in as hidden a place as possible on the furniture so that if the test is more aggressive, the remaining mark is not disturbing. It is best to start with less aggressive tests and gradually work up to eliminating all possibilities. If it is an older piece, prior documentation is welcome. The Internet provides a wealth of information and it is good to take advantage of this.

Ceara is soft and can be identified by its very low resistance to scratching (which can even be done with the fingernail) or abrasion. The edge of a coin is used to gently drag the surface. If a yellowish-white or brownish crumbly substance collects at its base, there is wax on the surface. Another test is to put a drop of linseed oil or mineral oil on the surface and leave for a few minutes. If the oil is absorbed and the film softens and becomes waxy, it is wax. Wiping the spot with a soft cotton cloth will leave dissolved wax on the cloth.

Old oiled surfaces have a shabby and dry appearance. If drops of linseed oil or mineral oil put on the surface are absorbed, it is definitely an oily surface. The oil droplet test is also valid for surfaces that do not necessarily look dry.

To see if the surface is applied shellac, nitrocellulose varnish or oil-based paint we do a few tests in succession. We put a cotton bud at the end in ethyl alcohol and then place it on the surface of the wood. If it's shellac it will dissolve immediately, whereas nitro varnish and oil paint will soften as time goes by and the place will become tacky. Repeat the test, this time dipping the stick in acetone or nitrocellulose thinner. Now the nitro varnish will be the one that dissolves immediately, and the others will soften over time. Finally we put the chopstick in a soda solution. This will liquefy the oil-based paint, the others will be unaffected.

The alcohol and acetone test can also be applied in the case of finishing with polyurethane varnishother chemically cured varnishes (catalyst added 50% and over) or UV-curing varnishes. The difference is that they will not be affected. In fact, such varnishes have very high resistances, with some resisting even if a cigarette is extinguished on their surface, without leaving a mark. Another test that can be done on such very hard surfaces is to scratch them with sharp and hard objects (knife). If it scratches very hard, and when it does, a white, brittle powder results, it may be a polyurethane varnish.

Water-based lakes are not affected by alcohol or acetone. Those without hardener, less cross-linked, become opaque and sticky if rubbed with a cloth soaked in nitro or universal thinner. Water-based varnishes with catalysts are attracted to more aggressive solvents such as toluene or xylene. If the surface wiped with these solvents becomes gummy, you are dealing with a water-based varnish. Beware, toluene and xylene are solvents considered dangerous.

Possible combinations when finishing with synthetic varnishes

There is a huge variety of synthetic paints and varnishes, and if you call a specialist from a company that sells professional products you will be asked a lot of questions about the project until they recommend the most suitable product. Products can be synthetic oils or waxes, varnishes and paints based on water, organic solvents or petroleum-based solvents, products based on different resins or a combination of these. Some of these materials can be applied on top of each other, others cannot. That's why by calling a professional company you will always get the complete system or at least recommendations for how you should use that product.

Polyurethane varnishes cannot be applied over nitrocellulose varnishes and I explained above why. But nitrocellulose ones can be applied over polyurethane ones. The condition is that the surface must be sanded before application so that the coats have adhesion. Being very resistant, polyurethane varnish is not soaked by the thinner in nitro varnish and varnishes do not stick to it. As a rule, hardening varnishes can be applied on top of each other provided that the dry coat is sanded first.

Water-based lakes can be applied over most varnishes and paints except alkyd and oil-based paints. Before application is mandatory sanding the layer to which it is applied for adhesion. Water-based varnishes have a very strong bond between the molecules so that if the varnish comes off in one place, it doesn't crack locally like polyurethane varnishes, but comes off just like sunburnt skin.

In the case of outdoor products, do not apply water-based varnish over alkyd varnish or water-based varnish over oil-based varnish. Nor is the reverse possible. Nothing more is applied to the waxed impregnation or wax waxing. For maximum resistance of exterior products, use the entire system recommended by the manufacturer (impregnating agent, primer, varnish, for example).

It is possible that sometimes, for various reasons, they have misapplied the materials and nothing has happened. It doesn't mean that the rule is wrong, but that you've had an exception. On the second application you may not have the same luck. It's good to follow the rules to have as few problems as possible and get quality finishes.

I hope you find the above useful. If you have finished wood and have had interesting experiences, I look forward to hearing about them. And if you have any questions or queries, leave them below in the space provided. I will certainly answer them.