The grout in my 1920s house is falling apart in a few areas near the tub. How can I patch just the damaged spots?
Riley Doty: Over time, old cement-based grout is likely to crack where the tiles meet an adjacent material or where there’s a change of plane within the field of tile (i.e., where two walls intersect). To repair it, use acrylic caulk, a material that’s water-soluble when fresh, but highly water-resistant after it has cured. While acrylic caulk isn’t as long-lasting or flexible as urethane caulk, it’s much easier to work with, emits virtually no toxic gases during application, and is readily available in a wide range of colors. (Modern tile dealers offer acrylic caulk to match 40 or so available shades of grout.)
Aim to produce a joint that closely matches the surrounding grout in color, width, and shape. Proper caulk selection will ensure a color match; careful taping controls the joint width; and finger-smoothing produces a slightly concave profile to match original joints. Before you start, gather all of your tools, create a mess-containment island by placing a dropcloth or cardboard square on the floor to house dirty tools, and keep a clean, damp rag handy to help wipe away messes.
First, carefully dig out the old grout or caulk, using a utility knife on joints 1⁄8" or smaller or a hand saw (available from tile dealers) for wider joints. An aluminum-handled Xacto knife with a #18 square-tip blade, used in conjunction with a hammer, makes an excellent precision chisel and carries less risk than a full-sized blade of marring adjacent surfaces. Work slowly and methodically. Vacuum or brush-clean the area, and wipe it down with an acetone-soaked rag.
Once the old grout is removed, carefully tape off the joint on either side of the tiles. The distance left between the bands of tape determines the exact width of the finished joint.
Next, use a caulk gun to apply the caulk, then press it down with a pallet knife and smooth it out with a gloved finger (wear snug-fitting nitrile or latex gloves). Wipe away any excess material from the tape with a putty knife, and smooth the joint a second time. These tooling stages should be done in rapid succession.
Finally, gently remove the tape, then make one more smoothing pass with a finger. For final touch-ups, gently run a clean, damp sponge that is well wrung-out across the joint. Together, these steps will pack the joints, clear excess material, and refine the joint shape for a like-new appearance. After the caulk has cured in a couple of days, you may need to wipe the tiles and tub with a rag soaked in mineral spirits to remove any haze or film.
Riley Dotyhas worked with tile for more than 30 years. He’s a member of Artistic License and sits on the board of the Tile Heritage Foundation.