Why is my grout full of tiny holes?

A pinholed joint looks acceptable when it is new and dark, and becomes obvious once it dries and lightens. It also holds dirt and moisture in every hole, which is why a pinholed bathroom joint discolours faster than a sound one.

Step by step

1. Mix slowly with a low-speed action.A paddle on a fast drill aerates the mix. Slow mixing, or mixing by hand for a small quantity, introduces far less air.
2. Let the grout slake, then stir again.Most grouts are meant to stand for a few minutes after the first mix and be restirred. That rest lets the powder wet through and lets some entrained air escape.
3. Do not over-water the mix.A slack mix holds air bubbles more readily and shrinks more. Follow the stated ratio.
4. Pack from at least two directions.Work the float diagonally across each joint, then again from the opposite diagonal. Coming at the joint from two directions closes voids that a single pass bridges over.
5. Press rather than wipe.The float should be pushing grout into the joint under pressure, not skimming it across the surface. Wiping fills the top and leaves air beneath.
6. Tool the joints lightly before they set.A light pass with a rounded tool or a damp finger compacts the surface of the joint and closes small holes. Do it once, at the right moment, not repeatedly.
7. Fill remaining pinholes with a second application.Once cured, a thin second pass of the same grout over the joints fills the holes. Work it in, then clean off carefully.

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