Just thought i'd post a followup in case anyone in the future has a vermiculite ceiling they need to spray. The quotes from painters to do the work was $750.
Anyway the ceiling l had to paint was about 30 sq metres. I borrowed my dads 20 year old wagner spray gun with 800ml capacity and set about spraying on 10 litres of dulux oil based sealer binder. This stuff worked like glue to set the vermiculite in place.
Two days later l got 10 litres of cheap ceiling paint (Wattyl Tradesman Hi Opacity Flat Acrylic) and tried brushing and rolling (extra long lambswool nap) it on. It didn't penetrate into the gaps at all so l had to revert to the wagner again. Spraying it on and then rolling back seemed to work really well and 10 litres did one coat. I put a second coat (another 10 litres) on top of this and it was all done.
All up 2 days work but total cost was only $210. $110 for sealer binder and $100 for 20 Litres of the Wattyl (yes it's a very cheap paint but had very good hiding qualities).
Anyway spraying the vermiculite lightened the room right up and makes it look much more modern and vibrant. I'd definately recommend doing it.
Brent