With any kind of metal or wood finish, it is not unusual to have to sand, scrape, or burnish between coats if you want to maintain a smooth finish. Of course, if you want a smooth finish, you must have a smooth finish to start. It takes an awful lot of finish to create a smooth finish from a rough initial surface.
In going for particular look, I will sand off a finish several times. If you want a uniform finish, you need to sand the old finish off uniformly, which may be almost completely. I think that really gets to the heart of your question. Also, as someone else said, make sure the surface is completely oil free, including from your fingers. Otherwise, you will get a blotchy result, which you may want in some cases. Just know that a smooth, clean starting surface is required for a smooth, uniform finished surface. You must also card uniformly between coats.