I have a good, solid marble slab that I would like to turn into a pastry stone. It is totally flat and level (no saw marks), but unfinished. It is totally unsuitable for cooking at the moment because it has a dull finish that chocolate and dough stick to it. It is not rough to the touch. It’s texture is probably like the top of a piece of granite curbing, as opposed to a polished granite countertop that should be the end goal
I assume that I need to sand it and treat it to turn it into a traditional pastry stone. But with what? I have a random orbital sander that would make it easy, but I’m not sure if the grits go fine enough. We also have sanding blocks around, so I can use those. But should it be sealed with something in the end?
You’re going to need some diamond polishing pads for the final buff, but I don’t know how you’re going to get it smooth enough to use those. They’re what we used to polish some concrete countertops we made, but those came out of the form already pretty slick.
An orbital sander will do a fair job. 80 grit, then 120, 240,320,400,500,600. Knowing when to switch to the next grit is the trick, switching too soon will leave marks behind that will not remove later, so take your time. Sanding wet will lessen dust and improve the result, but it is quite messy.
By the time you reach 400 it will start to shine. Beyond 600 is not really needed or practical. The orbital sander (or any handheld polisher) will leave inevitable swirl marks on any large surface. A slab fabrication machine ($150,000) uses diamond pads, ball bearings and oxalic acid with a very large polishing head to achieve a mirror finish on a large surface. You could buy small diamond pads to fit angle grinders or handheld marble polishers, but they are expensive and you will still have swirl marks regardless. The main advantage of diamond pads is speed.
The polisher we use is handheld, adjustable from 2000 to 6000 rpm with a semi-flexible velcro head and velcro sandpaper. I don’t know my paper very well, but it’s the black one (wet/dry), not the brown garnet one. The restriction is that the polisher (4" head) works well on edges, not on flat surfaces. It will do the job, but will leave swirl marks. A larger head will leave less swirl, but you better have some real torque or it will bog down to a crawl. Some of your success will simply depend on the type of stone.
Ideal rpm is around 4000. Torque is a must. Otherwise all it requires is patience. Wear earplugs! Eye protection not so much because you won’t be able to see the subtle changes. You need lots of light at the proper angle so you can see the stone come up to polish.
There are sealers, but its a food prep surface, so be a bit cautious. Subsurface silicone sealers will do well, surface type sealers won’t.
Wow! Thanks for all the info, Add99! After reading it all, though, I think that Projammer may have the right idea. I’m rather a clumsy oaf, and it seems like there is a lot of opportunity to mess it up by leaving swirl marks.