[LeighP]

Mate, don't sandblast the dash...they have a particular grain structure thats special to GM dashes of the period...you'd destroy that by blasting it.
To recolour the dash, scrub it with a cleaner like Ajax Spray and Wipe or dishwashing liquid and a soft scouring pad (worn scotchbrite), flush it with water and dry. Wipe with wax and grease remover using clean cotton cloths (lint free) and repeat till the cloth stays clean.
Very light coats of vinyl dye are the best way to build up the coats and colour depth.
Keep re-coating every 10-15 minutes (light coats) till you have the colour you want....at this point, stop. More coats are not better.
SEM dyes are hard to come by in Sydney....they also sell an adhesion promotor and a clear satin top coat.
I've had good results using VHT vinyl dye since I can't seem to find anyone selling SEM.