Hairline Crack in a Porcelain Sink: Repair or Replace?

A thin dark line across a basin can be one of three things: a scratch, a craze in the glaze, or a crack through the ceramic. Only the last one leaks, and only the last one means the sink has to go.

Short answer: Test whether the crack goes through by filling the basin above it and checking underneath. A crack only in the glaze can be cleaned, filled with a ceramic or epoxy repair kit and colour-matched. A crack that weeps, spreads or moves means the basin is failing and should be replaced before it splits.

Scratch, craze or crack

Scratch: a mark on the surface that you can feel with a fingernail but that follows no pattern. It never leaks.

Craze: a web of fine lines in the glaze, common on older basins. It is cosmetic, though dirt in the lines makes it look worse.

Crack: a single line that runs in one direction, often from a tap hole, the overflow or the edge of the bowl. This is the one to test.

Testing the crack

1. Dry the basin and the underside completely, using a torch to see the back.
2. Put the plug in and fill to just above the top of the crack.
3. Wait an hour, then feel and look underneath.

Damp, a drip, or a line that is darker than before means water is getting through. No change means the crack is most likely confined to the glaze.

Repairing a glaze crack

Clean the crack thoroughly with a degreasing cleaner and let it dry fully, ideally overnight. Ceramic repair kits contain a two-part filler that is pressed into the crack, smoothed level and left to cure, then a colour-matched glaze on top. Follow the curing time on the kit before running water into the basin. A good repair is close to invisible on a white basin; coloured and patterned basins are harder to match.

When replacement is the only safe answer

Replace the basin if the crack goes right through, if it is growing, if it starts at a fixing point where the basin carries weight, or if the basin moves when pressed. A cracked basin can split suddenly under the weight of the taps or a person leaning on it, which floods the floor and can cut whoever is nearby.

Replacing a basin means isolating the taps, disconnecting the supply and waste, removing the old basin from its brackets or pedestal, and fitting and sealing the new one with the taps and waste transferred or renewed. We charge £99 for the first hour and £49 per additional half hour, plus the cost of the basin and parts. Our guide to fixing a leaking kitchen sink covers leaks from the taps and waste rather than the ceramic.