Chuck,
Once again, this comes back to what you are after for the final result. The enamel that Ruth sells is easier to spray, sand, and buff than poly, but it is going to leave a visible line or halo where the new finish feathers into the existing finish. This is due to color match and the fact that you are dealing with two different materials. Because the enamel is softer, it is going to sand and buff "faster" than the surrounding poly (much like sanding a hardwood dowel that is glued into soft wood) so more enamel is going to be removed than poly. Also, the enamel will not perform as well in the long run because it is softer.
It is "possible" to repair this with another pour, but I wouldn't advise it. The exisiting finish is so thin that you will likely be right back where you started from after investing another 5 or 6 hours into the repair.
When you refer to the "front" of the fallboard, are you talking about the side with the logo? If so, this presents a challenge with spraying the entire side. If you can obtain a perfect color match, you can spray the entire surface and then sand it back until the logo is exposed. However, if the poly gods are not smiling upon you, there is still a high risk of sanding through the edges as you try to make the surface level.
If you decide to spray it with new poly, the best sprayable poly I've found in the US (without having to purchase a 50 gallon drum) is here:
http://www.esquirepiano.com/products.html. It's a little bit thinner viscosity than the poly we get for spot repairs which allows it to flow better out of a spray gun, and it is also a very high quality product. You may have to play around with tinting it with some of Ruth's Konig products to obtain a color match, or you might luck out and have a good match from the get go. I've never used this on Yamaha poly, so I don't know what you're up against as far as color match. I recommend spraying 3 coats and letting it cure for a minimum of 3 days before working it out. If you have the time, letting it cure for a week is best. Good luck and procede with caution because (as John already stated) Yamaha has figured out a way to obtain a great finish with very little material. So the danger of sanding through ANY area where the new finish is going to meet the old is extremely high.
Michael Spreeman
http://www.RavenscroftPianos.com
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