After a few rides, especially after a series of wet rides, your wheel brake surface will be covered in a black gunge, it’s a good idea to do some maintenance on them after.
In order to keep your rims working well, and wear down, clean off the surface of the brake track with hot soapy water. Then, get a cloth and some isopropyl alcohol, and wipe them down. You’ll be amazed at how much more gunk comes off.
Now, with the wheels off the bike, give the brake pads a look. Look for any embedded metal fragments, and pick them out with a sharp pick. Wipe them down with the alcohol, and if they look glazed, or smooth just take a file or sandpaper and rub them down. Also, while your here it’s a good chance to check just how worn they are.