Mac has done a lot of repair on his Yamaha so has an informed view on repairs. New keys may have better materials and/or design so would be ideal repair, but are expensive. Some parts of the keys may not be repairable from a practical perspective
One challenge selecting adhesives is identifying the plastic and coating type. Another is that the materials deteriorate over time, so may not react with adhesives as expected. Some adhesives could cause a failed bond or melt the plastics, for example.
-- For a start, the Japanese require a materials labeling stamp for plastics, which might help you identify the materials used on the key. I don't think it helps with coatings. https://en.wikipedia.org/wiki/Recycling_codes
-- You could call Yamaha and ask what type of adhesive they recommend. They probably won't answer you but it is worth asking.
Without seeing the key, I would try a 2-part epoxy "marketed" for plastics. You can look at the technical specifications to see which materials are n/a. You can test a small amount in a non critical area of the key to see if this is a complete disaster.
Depending on the crack, I might consider adding a "support beam" like they are using on the I-40 Missippissi bridge. But that might add a lot of weight or be useless. . .
Study the instructions and warnings of the epoxy. A few things I might consider:
- Roughen the bonding areas with a Scotch-Brite pad or fine sandpaper, then scrub with high % isoprop alcohol to clean (wear gloves so oils on skin don't contact the key).
- When epoxying, wear gloves and have great ventilation.
- Strict ratio of resin & hardener per instructions. Mix the two parts extremely well else get a lousy bond. Typically short pot life means that you should use small batches; don't use epoxy that is starting to get thicker, just mix a new batch.
Risks of DIY adhesive repair include:
- Destroying the key
- Bond fails
- New cracks emerge near the repaired area. Original breaks likely are in areas of physical stress, poor design, and/or poor materials.
- Depending of the area of damage, the weight of adhesives (and/or poor repair alignment) could impact the playing experience.https://dm.henkel-dam.com/is/content/henkel/tds-us-loctite-epoxy-plastic-5-min-final