We recently installed a limestone floor in our patio. Small dark spots of thinset and some thinset residue was visible after the floor was complete. The dark spots occurred because limestone is very porous and the thinset soaks into the stone. A mixture of poultice with a catalyst is required to remove the spots. Several applications may be needed if the spots are very dark.
To make the poultice, use poultice powder, diatomaceous earth or clay. Pour 1/2 cup of your chosen ingredient and pour it into a mixing container. Add 1/4 teaspoon of concrete film remover and 1/4 teaspoon of liquid grout. Stir the mixture together to create a thick, creamy poultice. Add more liquid as need 1/8 teaspoon at a time. The mixture will be similar to creamy peanut butter.
Apply a thick, 1/4-inch layer of the poultice over the thinset spot or filmy area. Spread the poultice out so it extends past the area approximately 1/2-inch. Multiple applications over the same area may be needed to create the 1/4-inch thick covering.
Cover the area with a piece of plastic wrap approximately 2-inches larger than the area. Position the plastic wrap so it is centered over the poultice. Secure all areas of the plastic wrap to the floor with 1 1/2-inch wide painter’s tape. Allow the area to dry for 24 to 48 hours. The poultice should be completely dry before you begin the next step.
Remove the plastic wrap and sweep up the dry poultice. Look at the spot or area. If it is still dark, reapply the poultice. Deep thinset stains may take up to five applications of poultice for complete removal.
It is possible that the cleaned area may need to be polished depending on your limestone’s finish. Polish the areas with abrasives or with polishing compound depending on the limestone’s original finish.