Slate is a natural stone, but unlike marble, limestone and granite, it requires different maintenance and troubleshooting procedures. The following are some of the questions I have heard regarding slate flooring.
1. What is the best way to clean a gray slate floor?
It streaks every time I mop it, and the mop seems to shed on the floor.It sounds as though your slate floor has a rough finish, which catches the fibers of the mop as it is dragged across the floor. First, you need to deep clean the floor to remove all dirt and grime. Next, apply a high-quality acrylic sealer. Several coats may be necessary. The acrylic sealer should make future cleaning much easier. Clean the floor with a neutral cleaner and re-apply the sealer as needed.

2. The slate steps on our front porch have developed a hard white deposit coming from the grout lines. How can we clean it off?
The white deposits are salts, which are bleeding through from the grout joints or from the mortar. An acid wash will be required to remove the salts. If the deposits are heavy, try scraping them off before washing the slate with acid. Several acid products are available for this purpose. Ask your local stone supplier.

3. What’s the best way to remove a polyurethane finish that was placed on my slate floor?
Unfortunately, most polyurethane finishes require strong solvent-based chemicals for removal. Methylene-chloride strippers as well as a number of safe strippers are available, but be forewarned: removing polyurethane is a messy job. I know several professionals who refuse to do it.

4. I have recently laid a black river slate floor. Having grouted the floor with the “slurry method,” I discovered that the surface of the majority of tiles had grout in the recesses and the cracks. Now the black slate floor has white lines across it. How do you rectify this?
While I do not normally recommend using acids on any slate surface, this is the one exception. Try washing the floor with a solution of sulfamic acid. You can find this acid at most home centers and tile supply stores. After rinsing the floor with acid, make sure to rinse with plenty of clean water.

5. I have a slate floor that has a few scratches from what looks like something being dragged across it. They’re fairly light, but stand out because they run the entire length of the floor. Do you have any suggestions on how to fix this? Will resealing the floor help blend them?
It is possible that a topical sealer would hide the scratches, however, to remove them entirely, the floor will need to be resurfaced. If the slate has a natural cleft finish — that is, if the texture of the slate is uneven — then the floor should not be resurfaced. Your only option would be to try and hide the scratches with a topical coating.

6. I have a slate floor that we just removed carpet from. We also rented a floor sander and sanded off most of the glue and paint from the slate. My original plan was to seal the floor, as is, but the carpet padding left a black tire-like imprint on most of the floor. It is very difficult to remove, and I’m wondering if a stain will cover it. Can I stain over this waffle print on the floor, or do I need a specific cleaner? I tried Jasco epoxy remover, but had no luck. While the sanding removed some of it, much of the imprint remains. Also, there is a large crack going across the floor. Will this seal okay?
Chances are a stain will not cover the pattern left by the carpet. I would try resanding the floor again with a more aggressive grit. You should not use a wood floor sander. I would recommend using a single disc 175-300 rpm floor buffer with diamond pads. This is a task for a professional, and I would consider hiring a professional contractor to do the work.