Grout Pinholes and Voids: What Causes Them and How to Fill Them

Grout pinholes are small air pockets, usually 1/64 to 1/16 inch across, that form when air gets mixed into the grout, escapes from the tile or thinset below, or is left behind because the joint was not packed tightly. Voids are larger gaps from under-filling or shrinkage. Small fresh pinholes can be filled with fresh grout. Larger or recurring voids mean raking out and regrouting the affected joint lengths.

Pinholes, bubbles, pits and voids: the vocabulary

Setters use these words loosely, but separating them helps with diagnosis:

• Pinholes are tiny, round holes with smooth walls, often scattered across the grout surface. Smooth, round walls mean a gas bubble formed them.
• Bubbles or blisters are pinholes that formed just under the surface and either broke open during washing or remain as a thin skin that collapses when pressed.
• Pits are irregular, rough-walled holes where sand or grout was pulled out, usually by the sponge.
• Voids are larger missing sections: low joints, gaps at tile corners or where the joint narrows, and hollows that open when you press on the surface.

What causes pinholes and voids

Air whipped into the mix

A drill spinning a paddle fast, or a mix run for several minutes, folds air into the paste. That air rises once the grout is in the joint and leaves round pinholes. Most data sheets call for a low-speed drill (commonly around 300 rpm or less) and a short mix, followed by a slaking rest and a brief remix. Keep the paddle submerged; lifting it near the surface pulls in air. The grout mixing guide covers paddle choice and timing.

Air rising from below (outgassing)

Air also escapes from underneath the grout. Porous tile bodies, unglazed edges, dry cement board and hollow pockets in the thinset all hold air. As the water in the grout soaks into these materials, it displaces the air, which bubbles up through the fresh grout. This is the same mechanism that causes bug holes in poured concrete. Telltale signs: pinholes that keep reappearing after you float over them, concentrated along certain tiles or in joints where the thinset coverage was thin.

Mix too wet

Extra water does two things. It makes the grout slump and settle as it loses water, leaving low joints and gaps at tile edges, and the water that evaporates leaves pores and shrinkage cracks. Wet grout also washes out easily, turning pinholes into pits.

Mix too stiff, or past its working time

Grout that has begun to set does not flow into the joint corners. It bridges across the top and leaves hollows underneath that open up later under traffic or a scrub brush. A batch that has stiffened should be remixed without water if the TDS allows it, or thrown away.

Poor packing

The float must force grout down into the joint, not just spread it across. Holding the float flat, or working parallel to the joints, lets the face of the float drag grout out of the joint. Proper technique is a steep angle (around 45 degrees or more) and diagonal passes in more than one direction. See how to grout tile for the full sequence.

Washing too early or too wet

A dripping sponge or washing before the grout is firm pulls out sand and opens up hidden bubbles. This produces pits and sandy pinholes rather than smooth round ones. It ranks among the most frequent grouting mistakes.

Joint geometry

Some joints are hard to fill completely. Narrow joints with a wide-grain sanded grout, joints squeezed nearly shut where tiles drift, pillowed or cushion-edged tiles with a deep undercut, handmade tiles with irregular edges, and joints partly filled with thinset all leave spots where grout cannot reach the bottom or cannot be compacted. Match the grout to the joint using the joint width guide, and clean thinset out of joints before it cures.

Epoxy-specific causes

Epoxy grout pinholes usually come from air whipped into the resin during mixing, from working in a hot room where the epoxy thickens quickly, or from aggressive washing that leaves small craters. Epoxy also outgasses over porous substrates as it warms. Mixing the full kit at the recommended speed and working at the temperature range on the TDS (often about 60 to 90°F) reduces both problems.

Read the holes: a diagnostic table

Do pinholes need to be fixed?

A scattering of tiny pinholes on a backsplash is cosmetic, and many will close up when the grout is sealed. On a floor they collect dirt and show as dark specks. In a shower they hold water and soap scum, stay damp longer, and become starting points for mildew (see grout mold and mildew). Grout is not a waterproofing layer, so pinholes do not cause leaks in a properly waterproofed shower, but they shorten the grout's life and make it harder to keep clean. Voids are a different matter: an unfilled joint lets tile edges chip and lets debris and water work into the setting bed, so voids should always be filled.

How to fill pinholes and voids

Timing matters more than technique. Fresh cement grout bonds well to grout that is still green (within the first few hours, and acceptably within about 24 hours). Once cement grout has cured, a smear of new grout over the surface has little to grip and tends to flake. For cured grout, the fix is either packing grout deep into the hole or cutting out and replacing the joint section.

1. Classify the problem. Use the table above. Tiny, shallow pinholes in sound grout can be packed. Voids, crusts and any hole wider than about 1/16 inch, or areas where the grout fails the key scratch test, need raking out instead.
2. Get matching grout. Use the same product and color, ideally from the same lot. If none is left, test a few shades on a scrap board and let them dry fully before choosing, since wet grout reads darker. The shading guide explains why patches can look different.
3. Clean the holes. Pick out loose sand with a dental pick or awl, vacuum, and wipe with a damp sponge. For sealed grout, remove the sealer in the repair area, since sealer prevents bonding.
4. Rake out larger defects. For voids and crusted joints, remove grout with a grout saw or oscillating tool to at least 1/8 inch (3 mm) deep, and ideally 2/3 of the joint depth. Cut back to the nearest joint intersections so the patch boundary is at a crossing point, where it is less visible.
5. Dampen lightly. Mist or wipe the repair area so the old grout and tile edges do not pull water out of the new grout. No standing water.
6. Mix a small, slightly stiff batch. Use the low end of the water range and mix by hand or at low speed to avoid adding new air. Let it slake, then remix. For epoxy, mix a full kit or a carefully measured partial kit; never eyeball the ratio.
7. Pack it in. For pinholes, press grout into each hole with a gloved fingertip, using a twisting motion to drive out air. For raked joints, pack with a small rubber float or margin trowel, then make diagonal passes.
8. Strike and tool. Remove the excess with the float edge held nearly upright. Once the grout is thumb-firm, shape the joint with a jointer or the rounded handle of a tool so it matches the profile of the surrounding joints.
9. Clean gently. Use a well-wrung sponge, light strokes, and as few passes as possible. The repair area is small, so it is easy to over-wash.
10. Cure and seal. Protect the area from water and traffic for the cure time on the TDS, then seal cement grout if your project calls for it. See grout cure time.

What not to use

• Caulk in grout joints. Silicone or acrylic caulk dabbed into pinholes looks shiny, attracts dirt and does not take sealer. Sealant belongs in movement joints, as covered in caulk vs grout.
• Thinset or patching compounds. These do not match grout color or texture and are not designed for joints.
• Colorant alone. A grout colorant can bridge hairline pores but will not fill real holes and can crack over them.

Preventing pinholes on the next job