The piano as furniture
The exterior of an upright piano or grand piano is naturally of great importance to you. The finish of the piano's cabinet is superior to any other varnished, stained or treated furniture. The type of surface treatment the factory decided to apply, determines also the kind of maintenance required. For this reason, giving general advice on maintenance of the exterior finish of your instrument is not possible, but the three most common types of finishes are discussed below, each having advantages and disadvantages.
Many older European, Japanese, and American pianos have a lacquer finish. These finishes usually have a satin finish. A thin lacquer finish is easier to apply by the manufacturer. However, it scratches easily, and although moisture resistant, it is not a good barrier to moisture. Never clean this surface with a damp cloth. Look after the cabinet as described in "French polished finishes."
Polyester is a resin that, when mixed with a catalyst, cures to a thick hard surface, which is impervious to water and many other chemicals. Since around 1960 most European and Asian pianos and some American pianos have a thick and hard polyester finish. Polyester is more resistant to scratching and seals the wood extremely well from moisture. These finishes usually have a very high gloss, and you'll easily see every speck of dust and fingerprint or the slightest scratch, especially on black. If the piano has a walnut or mahogany veneer under a clear polyester finish, you will notice less dust or fine scratches. Dust itself is highly abrasive and is like "sandpaper" to the very high gloss finish. If you want this finish to look new for as long as possible, never put books or anything else on top of the piano without having a soft cloth under it.
To clean the piano, first remove the majority of dust with a good quality static feather duster to which dust will adhere. Clean the feather duster of dust frequently outside your home and avoid disturbing dust inside your home, as you do not want air-borne dust in your home to land on top of your piano. Any remaining dust should be wiped off with a clean (flannel or cotton) dry soft cloth in the shape of a roll or ball and making a rolling motion with the cloth as you dust in a straight line, so that the dust is lifted up and away from the polyester surface. Never use a damp cloth, as it will leave ugly streak marks. Do not remove dust in a circular motion, as the dust will scratch the high gloss surface, and you will create a dust cloud. Shake the cloth clean outside your home after each stroke and repeat the process. Yes, it is a bit of walking to and fro, but the results will be great. Once your piano is as dust free as you can get with the soft clean cloth, polish the surface with polish, but not just with any polish.
Guus van den Braak sells amazingly good piano polish that gently cleans plastic keys and polyester finishes without scratching. It leaves a lustrous shine that resists fogging, repels dust, and eliminates static. It repels against smudges and scratching. Ask for a demo and you will be amazed. Polish with a soft cloth in a circular motion applying little pressure until all polish residues are removed. If a very fine scratch occurs now, it will hardly be noticed, because light that reflects from the scratch that is part of a circle is far less noticeable than light that reflects from a scratch that is in a straight line. Some pianos have a satin polyester finish. Although this finish looks really classy and it shows up less dust, be very careful polishing it. If you rub too hard in one spot, you may end up having a shiny spot. It is a very bad habit to place drinks, flowers or plants that need water to survive, on your piano, even with a soft cloth under it! Accidental water spills are extremely damaging to pianos.
There are disadvantages with polyester finishes. Polyester is a hard finish and may chip, shatter or crack like glass when knocked by another object. It cracks if exposed to temperatures near 4oC or lower, or if the underlying wood is swelling or shrinking too much. Therefore, keep your piano out of direct sunlight.
Repairing this kind of damage is extremely difficult and time consuming. Even the best repairer will not be able to disguise the repaired mark entirely when light reflects on this place. The polyester needed for the repair, often has a short shelf life and would need to be imported from the same piano manufacturer, as the colour would need to match. Not all black finishes are equally black. Some pigments are darker than others. Be therefore very careful not to scratch or chip the finish on your piano.
If the polyester surface has many fine scratches and has become dull, it is possible to have the surface buffed up to a very good and acceptable gloss, provided the polyester is in good condition.
French Polished finishes
Older pianos are traditionally French polished with shellac. It has been used for centuries in musical instruments and furniture. When many thin layers are rubbed on until the grain is filled, it creates a rich-looking, warm gloss, French polished finish. Polyester has a cold gloss, glass-like finish.
Unlike polyester, shellac allows moisture to permeate. French polished pianos are much more prone to veneer damage. It is a very bad habit to place drinks, or flowers and plants that need water to survive, on your piano! The condensation between vase or pot and woodwork is enough to cause great damage, even if you place a cloth under it. Alcohol will dissolve shellac; so keep all types of drinks off the piano as you will end up with white rings, even after a very short time. Accidental water spills are extremely damaging to pianos, as water will drip through the hinge, or over the edge, into the piano, causing a lot of internal structural damage that may cost thousands of dollars to repair. Shellac scratches more easily than varnishes and polyester, but can easily be repaired. Older French polished finishes are often in better condition than older polyester or lacquered finishes, and it is amazing how good a French polished finish can often be revived to look stunningly beautiful.
There are many commercial "polishes" and furniture oils available, but Guus van den Braak recommends you use none of them. Some "polishes" are nothing more than an oil/wax and/or chemical cocktail that can do more harm than good to your piano or furniture. All you would be doing is apply a "polish" that leaves behind a sticky residue of oil and/or wax, all which create a worse problem, as dust will easily stick to it. The good look will be short lived, and the next time you apply the "polish", you would be adding more oil/wax to the already sticky and dusty surface, creating layer upon layer. The best thing to do is to simply apply nothing at all and keep the finish dust-free with a soft cloth. Each piece of furniture should be assessed individually for the best treatment.
American-made pianolas usually have a spirit varnish that contains shellac. Over time, as the finish ages, it is likely to become very dark in colour and to crackle. There is not really anything you can do to repair this finish, other than totally stripping it off and applying a new finish.