I want to cast a black concrete worktop, once cured it will need to be ground (to expose the aggregate) and then polished back to get the shine. Trouble is, the concrete will not be hard enough to polish to a shine , so I need to bung in an addative. I've looked on the internet and if I was in USA I'd be spoilt for choice, but I'm not! Wickes etc sell a hardner but that I think is just an accelerator and not a hardner, what do you think?