I have a hearth made of soapstone with a "honed" finish which is a dull, matte kind of finish.
The mason who installed it got mortar on it, which stained, then when sanded off the mortar, the sanded area did not match the honed surface.
How can I restore the surface to a "honed" finish? I heard it is done with a "special" machine. Special in what way? Is there a particular grit that is used for a honed finish?
Honing is just the process that makes a surface flat. It doesn't usually leave a polished look, so you don't want that. What you want is really a slightly rough sanding to make the area uniform again.
I would try a random-orbit sander and a paper grit somewhere in the 150-320 range. If possible, try it out on a scrap or similar material to see if the degree of smoothness seems appropriate. Err on the side of too fine--you can always go coarser, but if you go too coarse you'll have deep scratches to deal with.
Press lightly and keep the sander moving slowly.
The finishing of stone or even concrete surfaces are done the a type of sanding pad made for that purpose. They can be used dry or with water to help the cutting action. Some pads can be used wet or dry, some dry only, some wet only. They are not cheap and they are used with "hook and loop" hard rubber backing pad.
Picture courtesy Amazon
The rubber backer is typically used with a variable speed right angle grinder. Using a regular grinder would be dangerous since the hook and loop pad will get thrown off at a VERY high speed. That could really hurt....
Using this system is about the only way you are going to get your "original look" back. Even though this is the same tools that the installers use, a "touch up" will still look different. Even before these slabs are cut to size, they are finished with a honed surface, and it is done with very large machines and water with the pads, not the little ones, BIG pads. There is a small chance it may tie in, since there will be nothing to loose, try detailing the immediate area.
Another thing to watch out for, if the sanding that was done to remove the masonry cement, was a more coarse grit than 400, you will need to start with a coarser grit as well to remove those scratches. Here is where the finishing is similar to wood. Use the next finer grit to remove the scratches from the prior cut, before moving onto the next finer grit. It will be a lot of work. I have had to do it a number of times, but it will get rid of the marks.
400 grit using these pads is considered a honed finish.
