I’m building myself a “La Cabronita” style guitar in black, with a very nice one-piece alder body from Phil at GuitarBuild.co.uk.
I can’t stress enough the need for thorough preparation for a guitar finish to turn out well. Black lacquer is very reflective and shows every defect, so it is especially important here.
Alder is a very absorbent wood and you’ll see it soaking into the wood, especially on the end grain.
I sprayed several heavy-ish coats until the end grain remained wet-looking, indicating that the wood had stopped soaking up the sealer.
Once I thought that I had plenty of sealer on, I let it dry overnight before block sanding.
Sanding showed up a few low spots and defects in the wood which I needed to treat.
The worst of them were a small knot between the control holes which could be seen as the sanding dust collected in it
and a depression in the edge of the body which remained shiny and unsanded.
I filled each of these with Ronseal 2-pack brown wood filler
and sanded smooth.
Afterwards, the whole body was given another coat of sanding sealer and allowed to harden overnight.
I wanted to be sure that I had sanded out and/or filled every imperfection, so decided to use a technique I learned from my car-spraying days called an index coat.
An index coat is a very fine mist coat of black (or another colour) lacquer which is typically sprayed over a primed surface.
You can even use this on transparent finishes because you sand it off again.
The index coat highlights imperfections because as you block-sand your guitar body, it remains in any low areas such as the very slight depression around our knot
and a few tiny dings which would have gone unnoticed.
I continued sanding until no black lacquer remained so I was sure that the faces of the guitar were perfectly flat and free of the tiniest of defects.
I sprayed another coat of sanding sealer and sanded to 320 grit, ready for the colour coat.