Hi, Steve, Two quick comments about your piano. The secret to a successful finish is Bartley paste wood filler. This fills the small pores so you can get a great lacquer finish. It is not made to fill major scratches. Use Bondo or Minwax wood filler for that. It is available at Woodcraft and, I am sure, other places. Follow label directions.
The second comment is that I hope you have a sprayer and a dust-free place to spray. Grand pianos normally have 20 or more coats of black lacquer, sanded every couple of coats. You will be wet sanding with 400 and then 600 grit wet/dry paper after you get a couple of base coats on. There is no question that you can do this and have a great finish, but it will be a big job. (That's why the pros don't want to do it. It takes a lot of man-hours and they may not have the people, the space, or wonder if you will pay their price when it is done.) Don't forget to use a good mask when you are spraying and that lacquer and the vapors are very flammable. Good luck. Henry in MI