Okay the first thing I would tell you is to be careful using black grout. It can stain your unpolished marble, your hands, and most anything else it touches. Definitely wear rubber gloves when you are working with it. If the joints are 1/8" than you can choose to use either sanded or unsanded grout, depending on whether or not you want a smooth or rough textured grout finished. Unsanded grout will not scratch like sanded grout can but since you have a rough rather than polished surface you should be fine. Since you are using black on an unpolished stone, I would recommend applying a sealer on your marble prior to grouting to keep the grout from sticking and/or staining the surface of your marble. A good one to use is TileLab Surface Guard penetrating grout sealer by Custom Building Products (available at Home Depot). Depending on the porosity of your marble, you may need two or three coats.
Beyond that I would just say make sure the grout is mixed to a toothpaste like consistency; It should not be runny nor should it be tough to spread. When you are ready to grout, start by grouting a hidden section, such as your closet to get a feel for what you are doing as well as ensure the grout is not staining your marble. If it is staining, you need to apply more sealer. Use a rubber grout trowel when applying the grout and wipe as much off the surface as possible before following up with a slightly damp sponge. Try to avoid spreading grout on the middle of the tiles, focus on the joints. Don't get too far ahead of yourself when applying the grout because you don't want the grout to set up too much and have to scrub it off. Have several clean buckets of water on stand by. You don't want to wait until your bucket of water starts to gather a lot of foam on the surface, which can discolor your grout. After you are finished grouting and washing, follow up with a clean bucket of water and sponge go get rid of any residual haze.
Oh, and do yourself a favor and before you start tape off any areas abutting the tile such as white painted baseboards which could be stained by the black grout.
Good luck and if you have any more questions I or one of the other flooring experts will be happy to assist!
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - Tile Productions
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