IF you are going to be using black lacquer use black lacquer primer. Mohawk makes it. Or use a gray lacquer primer. You CAN just use the black lacquer you are using for color coats - you have lacquer sanding sealer and black lacquer with chemically bond to it. DO NOT USE SHELLAC! It's sometimes used as a sanding sealer but it is not a primer OVER sanding sealer. All it will do is screw up the bond between the sealer and color coats. The only sanding from here on will be to fix minor runs. You don't want to surface sand between coats - it's redundant on lacquer and can introduce contaminants. And don't sand between color and clear for the same reason - you want the coats to melt into each other properly. Black is the most difficult single-color opaque finish to apply well, so practice on scrap until you have your application technique *nailed*. The piece needs to be hanging or attached to a paint stick so you can spray 90% of it at 90 degrees to the surface at 8-10" If it lays flat you have to shoot at an angle and the coverage will be [very inconsistent. Each coat should consist of 3-4 VERY light passes at a minimum, and a single coat will not cover very well. If it does you are applying too heavily, which is much worse than applying too thin. The black is applied that way; with clear you apply the first 4-5 coats like that and the last one or two can be "flood" coats to create an even, smooth, glossy finish. Again, this takes PRACTICE on scrap wood. When you have applied the black reasonably smooth and clear coats laid out smooth and even (where you can go straight to buffing - surface sanding is only done if the finish was applied unevenly). THEN start with the color coats on the guitar. Edited to add - pigmented lacquer primers are used as primers in SOME situations - as a buffer coat between a polyurethane and an enamel or lacquer, on bare (or filled but NOT sealed) wood that has knots or is known to bleed tannic acid and on difficult to coat metals, plastics and other adhesion-rejecting materials.