You have a partial blockage downstream of where the shower drain & the sink drain join into one pipe. You could try a thin plumber's snake, or add some drain cleaner, some bleach, or vinegar. About a liter of vinegar is the safest and should work well enough.
Davis
(davisbigelow.blogspot.com)
Bathroom sink use 5 liters of water.
Yes, they work in either.
In my experience, I would say no. Several reasons can prevent the overflow from working. Since the overflow of the sink is rarely used it has a tendency to be blocked at the lower portion where the sink joins the drain pipe. During normal use and over years the holes in the sink drain at the point where the overflow and drain pipe meets has a tendency to clog making the overflow drain slowly. You can notice this when your sink drains with a "gulping" sound. You can test the overflow drain and see how obstructed it is, but do it slowly and do not turn on the faucets on full.
I found someone's Medabon on the bathroom sink.
use slicone glue
See the link below.
You could use a bathroom and use hot water :)
Bathroom sinks use 1 1/4 pipe for the downspout in the sink. You can use 1 1/2 from that by using an adapter washer in the connection above the trap.
taking a bath and using the bathroom and shower.
If it is long enough between uses for the water in the trap to evaporate then sewer gas can work it's way past the trap.
Product you should use when trying to unclog your bathroom sink are products like drain-o. Drain-o works the best in my opioin. When you buy off brands they do not work as well.
Some ways you can remove nail polish from the bathroom sink is by using baking soda and lemon juice. And you can just use nail polish remover too.