A sink that weeps at the rim is usually the seal
I put a bowl under the kitchen sink in March and still found a wet stripe on the cupboard floor the next morning. I thought the waste had split. The bottle trap was dry. A bottle trap that weeps is usually the washer, and that washer was seated. The drip was coming from the rim, a slow bead on the underside of the bowl, then a run down the front panel onto the shelf. I had recaulked that sink two summers back after a stain at the tap hole. I had wiped the old silicone with a cloth and laid a new bead on the smear. The new bead had pulled away from the worktop. Wash water was sitting in the gap and tracking under the bowl.
A sit-on sink, or an inset one, sits in a hole cut in the worktop. The bowl is not watertight to the timber. The only thing that stops washing-up water going into the cupboard is the seal under the rim: a bead of silicone, or a ring of plumber's putty, between the lip of the sink and the top face of the worktop. If that seal has shrunk, split, or been smeared onto dirt, it is not a seal. Water from the tap, from the cloth, from a pan tipped too fast, finds the gap, follows the underside of the bowl, and shows as a drip at the front or a dark patch on the chipboard edge. People blame the trap because the cupboard is wet. The trap can be bone dry while the rim weeps.
I checked it with the cupboard empty and a dry rag on the shelf. I ran the cold tap into the bowl and watched from below with a torch. Nothing at the waste nuts. A bead formed at the front lip within a minute. That is the test. If you cannot see the underside, you are guessing. I dried the rim from below, ran the tap again, and the bead came back in the same place. The silicone on top looked intact. From underneath it had a split you could catch a fingernail in, and a greasy film on the worktop where the old putty had been left in place.
Restore is a clean seat and a new bead, not a smear on top of a smear. I isolated both supplies under the sink and opened the tap to prove they had stopped. If an isolation valve that will not close is usually the slot, and yours will not close, I stop. I do not open a live supply I cannot isolate, and I do not lift a sink on a live feed. Waste off at the trap, taps off at the tails if the sink has to come up, then lift the bowl. I do not re-pipe. Old silicone and old putty come off the rim and the worktop until the seat is bare. A wet or greasy seat will not take a bead. I let it dry. New silicone, or new putty if that is what the sink was set in, in one continuous run, no gaps at the corners, then the sink goes down square and stays weighted until it has gone off. Excess that squeezes out gets tooled, not smeared back into the joint.
If the worktop edge is swollen or the chipboard has gone to mush, I stop. A new bead will not hold on rotten board. If the bowl is chipped through at the rim, I stop. If I cannot isolate, the sink stays down and the job waits.
Skip laying new silicone on the old skin. The new bead has nothing to bite and it will peel in a season. Skip plumber's putty on a wet, greasy, or dusty rim. Putty needs a clean seat the same as silicone does. Skip a bead on the underside only. The water is coming from above the worktop. Skip lifting the sink with the supplies still live. Skip a jubilee clip, tape, or bathroom mastic as a substitute for a rim seal. Tape is not a seat.
This shows as a stain on the shelf, a swollen front edge on the worktop, or a smell in the cupboard long before you see a drip. After Christmas the sink sees more pans and the old bead lets go. In a cold snap the cupboard stays shut and you notice late, the same as any other slow weep under a bowl.
I do not need a new sink because the rim wept. I need the old seal off, a dry seat, a bead that actually meets the worktop, and a stop if I cannot isolate or the board has gone.
If I later add a buy link here, it will be a paid promo.