Do it Right the First Time
Unlike a piece of furniture, a piano needs a lot of extra care and special handling when it is refinished. It must first be carefully disassembled, and not a screw or bolt can be misplaced or lost. The strings, hammers and keys must be protected from solvents, stains and overspray. Any damaged spots must be repaired, whether dings, missing or loose veneer, burn marks, etc.
Different species of wood are typically used on the same piano, depending on their purpose, but different species take stain differently. Therefore, great care is required when staining. All the parts must match in color. The grain should be enhanced, not hidden. Choosing the wrong type of stain will actually obscure the beauty of the grain rather than enhance it!
The pores must be filled with special filler to give the finish a totally flat, flawless surface. Not doing so results in a piano that looks like it was refinished by an amateur. Many furniture refinishing shops ignore this critical step when trying their hand at refinishing a piano. Multiple coats of lacquer are required in order to protect the wood and increase the clarity, or "chatoyance" in the grain.
The finish is then "rubbed out" and polished in the manner traditional to piano factories for the last several hundred years. This labor-intensive process gives the surface a flawless look and feel and is the final thing that really distinquishes a piano finish from most other furniture finishes.
How Long Will a Piano Last?
As long as someone loves it and will have it refinished when it needs it, and will replace strings, hammers and other parts every 40-60 years or so, and it's not exposed to destructive swings in humidity, the lifespan is somewhat indefinite. There isn't any reason, why, with proper care, your precious heirloom piano can't still be a gorgeous musical instrument 150 or 200 years from now! Given that perspective, it makes sense to refinish your piano when it needs it.