Keep Grout an Even Colour by Waiting 24 Hours and Mixing Only What You Can Use

The Problem

Grout is the part of a tile job everybody sees, and it's where a good tiling job goes visibly wrong. The tiles can be flat and the layout perfect, and blotchy grout still makes the whole wall read as cheap.

Two of the three common causes are about timing rather than technique, which is why they catch careful people.

The Techniques

Wait 24 hours after tiling

Thinset holds a lot of water and gives it up slowly. Grout the joints while it's still green and that moisture migrates up through the fresh grout as it cures, carrying pigment with it and leaving the joints patchy — light in some places, dark in others, in a pattern that follows the thinset rather than anything you did.

Once it's blotchy it stays blotchy. Colour variation from moisture is in the body of the grout, not on the surface, so it doesn't clean off.

Mix small batches

Mix only what you can use in 20 to 30 minutes. Past that, grout stiffens in the bucket, goes stringy, and stops packing into joints properly — it bridges the top of the joint instead of filling it, which looks fine wet and shows as a hollow or a crack once it dries.

Never add water to grout that's thickening

This is the one that feels harmless and isn't. Grout stiffens because cement hydration has begun — the chemical reaction that makes it hard has already started. Adding water and re-mixing breaks up the structure that reaction has built, and the grout goes on to cure weaker and paler than the rest of the job. You get cracking, and you get colour that doesn't match the batch either side of it.

If a batch has gone off, throw it and mix a fresh one. That's cheaper than regrouting a wall.