The biggest problem with most bathroom sinks is that people generally don't "flush" the trap with a sufficient volume of water after using it.
This leaves toothpaste foam, soap scum, hair, shaving cream, etc, to coagulate in the P trap.
Talk about yuck factor, it's a slimy mess....
The best thing to do is to frequently close the drain and fill the basin with hot water and then let it flush.--
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