Finish Repair Overview

Updated: Jun 29

Sponsored by Allied Piano

By Gregory Cheng, RPT

A lot of piano technicians are afraid of finish repair. You should not be. While it does require practiced skills, patience, and attention to detail, finish repair is simply about understanding basic fundamentals of coloring, filling, sanding, and polishing.

There are many concerns about the health ramifications of finish repair, but proper handling and use of masks and gloves negates most of these concerns. Working in a well-ventilated area is also recommended. Wet sanding is required for finish repair to lubricate the sandpaper and remove the sanding dust. This in turn keeps the particles out of the air. Yes, the polyester and lacquer may smell bad, but as long as you are not huffing or drinking the stuff and have good ventilation with an open window you should be fine.

There are three types of modern finishes: polyester, lacquer, and polyurethane. Polyester is the most common in all grades of pianos today. Lacquer seems to be phasing out as time goes on, and polyurethane is very uncommon. Having the skills to identify types of finishes will tell you how to proceed since they are not generally cross-compatible.

Polyester is a hard, durable, but brittle type of finish. When repairing a polyester finish, you are essentially over-filling a patch of fresh polyester and sanding flush to seamlessly meet the edges of the old finish.

Lacquer is a softer, more fragile finish. Lacquer is far easier to work with since the lacquer melds into itself. Because liquid lacquers are solids in a carrier you have to over-fill and sand flush. However, over time, lacquer can shrink as the solvent evaporates out of the lacquer. Time is your friend in this case. Being able to patch the depth of a repair using epoxy putty or wax burn-ins will help reduce the amount of shrinking and the amount of lacquer required.

Polyurethane is a combo of both lacquer and polyester and usually can be matched with a lacquer topcoat. Deep repairs are often filled, faux grained with lacquer and sprayed out with lacquer to match the sheen.

The sheen of the piano will also play a role in what level of sanding to end at before “applying” the sheen. For instance, a high-gloss polyester finish will require you to go up to 1200 grit sandpaper, then through compounds and polishing pastes, or a buffing wheel to achieve the high-gloss mirror-like finish. Satin pianos stop at 600 or 800 grit sandpaper and then use steel wool to add the satin finish back into the piano. For satin luster pianos, which are becoming rarer, a combination of steps including steel wool, compound and polishing pastes will provide that shiny luster with the subtle satin finish.

There are two main rules when it comes to finish repair. The first is to under-promise and over-deliver. This should be the rule all of the time for all piano work. If you set high expectations for your work then you are guaranteed to fail from your client’s perspective. With the appropriate skill set, training, and experience in all cases of piano work, you will always come out looking like a hero when the job is completed to satisfaction for both you and your client.

The second rule is to watch your drips. Wet sanding without attention to detail will be messy.  Dirty drips of sanding dust on the carpet or in the piano cause further damage to the piano and is unprofessional regardless of how good the finish repairs are. Use pads and drop cloths to protect the area you work in. Tape off parts of the piano that may be affected in your workspace. As field technicians, carrying pads for our tools protects our client’s property and gives us a dedicated work area.  

Use the acetone test to determine which type of finish was used on the piano. On a discrete area of the piano (the inside back of a leg, under the lid on the bass side rim, or the back of an upright) wipe a towel that has a small bit of acetone on it. If the towel shows black or clear residue it is lacquer; if it is clean it is polyester; and finally, if the towel is clean but the finish is slightly distorted it is polyurethane. With experience, you will be able to tell what kind of finish you are working with by eye and feel without having to use the acetone test.

Polyester Repair

This is a chip on a Steinway keybed above the leg:

I used a business card to highlight the damaged area and give an approximate size reference.

I taped off the leg to protect it while I was preparing the damaged area for new polyester:

Notice how the rough edges of the damaged area are now smooth and the entire area is concave. This is done so that the polyester can be filed over. Over-filling creates a blended effect when polished. Unlike lacquer, polyester doesn't actually blend.

I color-matched, created a dam, and filled the damage:

Notice that I used additional tape to protect the undamaged areas from future steps of sanding and leveling.

This is the result after the polyester cured and hardened:

Notice how it is overfilled the damage. Next, I level this flush with the existing finish.

I removed the tape after leveling down far enough because it is actually higher than the existing finish:

Once at the finish level, I wet-sanded progressively through 600 to 1200 grit, then continued on with compounds or a buffing wheel:

This is the result after careful sanding through grits 600-1200:

Notice how the damage is now indistinguishable from the sanded finish. Now it is time to remove the sanding marks and bring the finish to a high gloss shine.

After buffing and polishing, the result was nearly flawless:

Polyester Repair 2

This is the same process on a Steinway D lid that was damaged when it hit the side of stage:

I taped off the repair area and smoothed out the edges:

Notice how much bigger the damage actually was once all the broke polyester was removed.

This is the result after filling, leveling, sanding, buffing, and polishing:

As good as new!

(The marks you see in the picture are actually the reflecting of the ceiling. It is hard to photo a mirror-like surface.)

Lacquer Repair

This repair is from about 20 years ago. Thankfully the client had the veneer that had broken off:

After I glued the veneer back into place, I taped off and covered the non-working area and used the old-fashioned method of combining powder pigments and nitrocellulose lacquer to color match and faux grain:

If the color touch up box provided by Allie