There are several techniques you can employ to smooth out a rough finish. Regardless of how much preparation you put into your painting surface, there may still be impurities at the end. The good news is, these can be easily sanded and wiped out or even painted over to give you a perfectly smooth finish.
The best way to remove large impurities is to sand them out. Use a piece of large general use sandpaper and gently rub a specific area in a circular motion. Do this over all irregularities until the surface feels smooth under hand. Simply wipe away any sand and dust debris before painting another layer.
Use a Cloth
Using a cloth will remove all impurities sticking to the surface, which prevent it being smooth. It is vital you wait until the paint or any layer you have already applied dries first. Dampen the cloth with some warm water and wipe over a specific area using a heavy amount of pressure.
Use one of the above techniques to get a level surface before you repaint. Apply a thick layer of clear coat primer with a large paintbrush. Wait for this to dry and use another large paintbrush to apply one or two more paint layers.