As far as rocks go, slate is fairly porous. That can make paint removal problematic because the metamorphic rock is too soft to clean with a pressure washer or sand blaster, which are go-to remedies for removing paint from brick and concrete, and it scratches easily. If you're just trying to remove a few paint spatters, heat, scraping and slate-safe solvents are all options, but if you're removing paint from a large surface, such as a floor, you need to use a chemical stripper. The procedure is messy, and -- depending on the paint -- it usually involves two stages and two different chemicals.
Removing Paint Splatters
While acidic cleaners are generally not recommended for slate because they can etch natural stone, common paint solvents are safe, provided you use them in moderation. Isopropyl alcohol softens latex paint, allowing you to wipe it off or scrape it with a plastic putty knife. If the paint spatters are oil-based, you should be able to remove them with a rag soaked with mineral spirits, acetone or lacquer thinner.
When the Slate is Sealed
Slate floors, countertops and other surfaces are often sealed with a clear urethane finish, which keeps the stone shiny and protects it from moisture. There's always a danger of damaging this seal coat with solvents you use for removing paint spatters. In some cases, softening the paint with a hair dryer makes it soft enough to scrape off without damaging the seal coat, and in others, scrubbing with mild detergent and water may do the trick. If neither works, use a solvent and touch up the sealer when the paint is gone.
Stripping a Slate Surface
You remove a coat of paint from a slate surface, such as a floor or countertop, just as you remove paint from wood -- use a chemical stripper. Because slate has an irregular surface, however, scraping is more difficult and less efficient. Because of this, slate professionals use a second procedure to remove residue; this procedure involves a mild alkaline stripper and a buffing pad.
Spread a commercial paint stripper on the surface, using a paint roller. Products containing methylene chloride work fastest, but if you want to avoid the fumes and possible skin burns, choose a citrus- or soy-based product. If you do, be prepared to wait as long as 24 hours for it to work.
Wait for the paint to start bubbling. If any of the stripper dries out before this happens, apply more. If you're using a stripper that takes a long time to work, it may help to spread plastic sheeting over it to keep it moist.
Scrape off the paint using plastic paint scrapers and putty knives. Using plastic implements allows you to scrape aggressively without fear of scratching the slate. Deposit the old paint in bags for disposal.
Neutralize the stripper by rinsing the surface with clean water.
Mix a commercial alkaline stripping solution with water in a bucket, following the instructions on the stripper container.
Spread the solution on a section of the slate surface -- use a string mop for floors and a sponge for countertops. Be sure to wear goggles and rubber gloves.
Let the solution dwell on the surface for 10 to 15 minutes to give it time to seep into the pores of the rocky material. If any area dries out, apply more solution to keep it wet.
Scrub the surface with a buffing machine. Use a floor buffer and a black stripping pad for floors -- you should be able to get the pad from the outlet that rents you the buffer. If you're stripping a counter, use a hand buffer with a stripping pad. Scrub until the paint residue breaks up and comes off.
Pick up the stripper and paint residue with a wet/dry vacuum cleaner. Put it in buckets and dispose of it as hazardous waste -- don't pour it into a sink drain or toilet.
Rinse well with clear water, using a separate mop or sponge to spread the water. Pick up the water with the vacuum. Repeat if necessary.