checking neck pocket flatness on an electric bass.

[arie]

So I'm restoring a Warwick Streamer 5 string electric bass. Some shade-tree hack burrowed into the neck pocket floor like a beaver and then tried to cover their tracks with super glue, paint, and masking tape. The "dowels" they poked into the neck tenon pulled out and burrowed into the pocket on their way to stripping out. The bass came in with horrible action and a large visible gap in the neck joint. I've scraped and block sanded and checked with a square against a light and now the pocket is pretty flat and the neck tenon has been properly plugged and delt with.

I'm ok with the work so far, but I have a obsessive-compulsive precision problem and just want to know that the mating surfaces have maximum flatness.

When I was a tool and die maker we'd check for mating flatness when lapping by using Prussian Blue, the modern version is basically ever-lasting, ever-spreading, ever-messy blue ink. Obviously a no-go on a guitar.

My question is: are there any products, substances, etc,, one could use in such a manner? Something I could apply to say the tenon of the neck and then mate it to the neck pocket and leave some indication of where the surfaces met? so I could work down the high spots? I have heard that chalk can be used but have not tried it yet.

Usually I'd either apply some psa to the neck tenon to bed the pocket down with, or just do a dry rub and look for the burnished areas -but this bass has a pocket the walls of which follow the taper of the neck and as such, the neck only lifts out vertically.

thanks,