How to Grout a Mosaic

Sanded grout is the right choice for almost every mosaic. Unsanded slumps out of the joint and tends to crack as it cures, and the packet will tell you it only suits very narrow lines. Sanded holds its shape, resists cracking, and fills wide and narrow gaps equally well.

Tip the powder into a plastic cup, and be generous. Stopping halfway through to mix a second batch means the first is already setting, and if you are tinting your own colour you will almost certainly never match it again.

Add water a little at a time and stir. You are aiming for the thickness of peanut butter. Too wet and it slumps straight back out of the joints, too dry and it will not pack in. Add more powder if you overshoot on water.

Work the spoon right around the sides and across the base of the cup. Dry powder hiding at the bottom is the usual cause of weak, crumbly patches that fall out of the joints a week later.

Leave the mix to stand for 5 to 10 minutes so the chemical reaction gets going, then give it another stir before you use it. Skipping the slake gives you grout that looks fine going on and cures soft.

Raised or delicate tesserae, and anything with texture that grout will lodge in, gets covered before you start. Tape or a scrap of plastic over the top works, and it peels off clean at the end.

Gloves on, and get your hands in it. Push the grout across the joints from several directions rather than spreading it in one pass, so every gap packs full. Take your time. It is messy, and rushing it leaves voids.

An old stiff brush pushes grout underneath raised tesserae and into corners a finger cannot reach. The brush will be ruined by the end, so pick one you were going to throw out anyway.

Scrape the surplus back into the cup and check the edges for gaps, filling any you find. Wash your tools outside or in a bucket. Grout sets like rock, and rinsing it down the sink will block the trap for good.