A slow-draining bathroom sink is something everyone has to deal with at least once in their lives. These partial clogs are annoying, but the fact that they still drain eventually is good news. The easiest way to deal with these is to call a plumber, but of course, that costs money that you might not want to spend. There are a few things you can try first, but be aware that calling a plumber may be necessary anyway if these don't work. Also, be aware that you should never use a chemical drain opener. If you use one and it does not work, then you have a sink full of water and caustic chemicals. Even if that drains away eventually, you now have this chemical coating over everything. It's better to try other remedies. 

Use a Hair-Removal Tool

First, many clogs are caused by hairs that have fallen down the drain. The hairs can get tangled up and form a mass that eventually starts to block the drain. Even short hairs from shaving can do this eventually, so don't assume that because you're not brushing your hair over the sink that nothing like that is going down the drain. You can now buy inexpensive hair removal tools to pull the hair out of the drain. Some of these tools look like plastic strips with spikes on either side, while others have a hook-and-loop type of material at the end that catches the hair. These are flexible and can often fit in the space between the sink drain edge and the stopper. However, if possible, it's better to remove the stopper before using this. Many times, removing the hair in the drain is all that's needed to open the drain.

A Gentle Drain-Opening Concoction

If you pulled up hair but didn't manage to open the drain, or if you couldn't pull up any hair, the partial clog could be due to buildup on the sides of the drain pipe of soap scum and sludgy biofilm. Trying a mix of salt, baking soda, vinegar, and hot water might work on this. When the water has drained out, pour coarse salt and baking soda into the drain – recipes vary but tend to be around a half cup of each or so, and you can leave the salt out if you want – and follow it with white vinegar, closing the drain. Wait only a few minutes; you really don't need to let it sit for hours (and in fact, you shouldn't, as that can allow any leftover baking soda to dry and cake up, making the problem worse). Pour a few cups of hot, but not boiling, water into the drain.

Try a Grease-Dissolving Dish Soap

If you prefer, or if you know that you use a lot of oil-based cosmetics, you might try a grease-fighting dish soap. You want something known for dissolving grease. Pour some of it into the drain and let it sit for a few minutes. Follow it with a few cups of hot but not boiling water. Don't be surprised if you see the dish soap lather up a bit; you can pour more slowly if that helps.

If none of these solve the problem and you still have a slow-draining sink, then it's time to call a plumber. Luckily, solving bathroom sink drain issues tend to be simple and does not take that long.