Kitchen sink: on top, under the worktop, or both?

Water finds the cut edge

A kitchen lives with fat, washing-up and almost constant moisture. An elastic, neutral bead that copes with cleaners — OTTOSEAL S100 here, or a stone silicone if the worktop stains — is not “any bathroom tube”.

Inset (the sink sits on top): the critical joint is the top flange against the worktop. Miss it and water creeps between flange and cut edge.

Undermount: the joint that saves the core is underneath, between the bowl flange and the underside of the board. A pretty bead on top and nothing below leaves chipboard open to the washing-up. The edge swells, rots and the bowl lets go.

The visible line alone is not enough. And if the cut-out is already rotten or soft, I do not rebuild it with silicone: worktop or timber repair first.

How I do it

1. Cut out all the old sealant, without wrecking the edge.
2. Degrease, dry, dust-free. In a kitchen the flank is often greasy: skip the degrease and you get no adhesion.
3. Primer only if that substrate needs it (some stone, porous timber).
4. Undermount: a continuous bead underneath, pressed home. Inset: geometry on the top flange.
5. Tool at once. Keep water off for a day or two, depending on depth and product; that is not a universal timer.

If the meeting is deep, backer rod or bond-breaker tape: three-sided adhesion kills the stretch.

Mistakes I see

Forgetting the undermount. Sealing onto grease or wet. Gun fresh silicone over the old. Aggressive kitchen cleaners on a fresh joint.

Natural stone: stone chemistry, or you get a halo on the edge.

Exceptions

A swollen core, a cut-out that will no longer take screws: joinery. A skinny undermount that was cut badly is not “filled” until the bowl actually sits.