Like investment plaster the killer for a good finish is the porosity
so when you make your mixture I would advise on not skimpimg on the
water but degassing the slurry by vibration or better still, vacuum.
The chemical reactions that make cement hard take place over time so
I would recommend leaving your cast piece for 28 days before
working/polishing. I wouls also say that you need to add some silica
to your OPC, very fine acid washed sieved sand is best, and for a
bit more texture crushed marble doeswell. The growing calcium
trisilicate crystals bond really well to the very fine sand and give
you a stronger solid than just adding water to the OPC and letting
it set. Try very fine crushed glass (almost as a dust) as an
alternative to sand for body colour. Bullseye Glass do some really
good powdered glass for casting and fusing in a suitable sieve size.
Nick Royall