The top veneer of newer concrete never really hardened into a surface I could polish. It remained weak and grainy, so I scraped it all back off:
In removing the little PVC segment i'd used to cast a hole for the gas key, I chipped more of the surface. Vexing!
I gave it some sanding afterward, but because I'd given up on it looking "nice" I didn't go beyond 200 grit polishing (and I had pads going up to 3000!).
I called friends to help me carry it (300+lbs) inside. I had to cut a bunch of the wood to get it to fit right -- the standard approach of measure once, cut 5 times -- but it's in. I didn't wind up gluing or shimming it at all -- it's pretty solid.
This was a frustrating project, but the flaws aren't as offensive in person as I feared they would be. They're less obtrusive than I'd feared, and all through christmas people were sitting on the hearth, so you couldn't see them anyway.
The overall effect is fine -- I'm not in a huge rush to replace it like I thought i'd be. I could do this one a LOT better if i were doing it a second time around.
Notes to future self doing a similar project:
- casting it in place is a bad idea -- this is a messy operation.
- casting upside down on melamine worked well.
- make sure the form is well-supported - the weight will want to bow the form.
- mortar (as a material rather than true concrete) is fine.
- reinforcement could probably be the heavy wire mesh instead of rebar, probably doesn't matter.
- wet polishing is awesome, don't dry sand it.
- I couldn't tell if additives did anything at all.