Ok I said I wasn't going to, but SA Color's primer/sand/filler is crap
Used up the entire litre, and could not get anywhere near where I was happy.
Sanded a heap off, including the lumps that came from a brand new supply, and drove to Super Cheap Auto and purchased primer/filler off the shelf.
This had zero lumps, mixed beautifully and applied even better. The cabinet I think looks great, and is now ready for top coat.
When I compare the 2 products, it was chalk and cheese - now I have my fingers crossed the black acrylic from SA Color (already purchased!) doesn't cost me another trip to SCA - tomorrow will tell!!!
Thanks again for all the advice - cheers