How do you unblock a sink with standing water in it?
The short answer
Bail the standing water out first — you can't plunge effectively through a full bowl, and you don't want to be splashing chemicals about. Then block the overflow with a wet cloth, seal the plunger over the plughole and work it firmly a dozen times. If that fails, put a bucket under the trap and clean the trap out by hand.
Blocking the overflow is the step everyone skips, and it's the one that makes plunging work. If you don't, most of the pressure you're generating just escapes out of the overflow hole rather than pushing at the blockage.
Plunge in firm, deliberate strokes, keeping the seal, rather than thrashing at it. You're trying to shift the blockage with pressure waves, and the water needs to stay in the pipe to do that.
If it stays put, go to the trap. Bucket underneath, unscrew, clean it out. Between plunging and cleaning the trap, most kitchen sinks are back in service.
Don't add caustic cleaner to a sink that's already full of standing water. It sits there, it may not reach the blockage, and it turns a simple job into a hazardous one for whoever opens the pipe next.
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