With a few good painting tips, most surfaces can be painted regardless of the condition. Painting over filler can be a little tricky. If you've painted over a filled area and notice the filler is showing through the paint, you can follow these simple tips to correct it.
Step 1 - Sand the Area
The first thing you will want to do is prepare the area by sanding it down. If the filler is uneven, this is going to show up through the paint. You shouldn't need to sand a large area, just over the filler and then about an inch around. You should be able to sand a small area by hand, remember to switch the kind of sandpaper to get a smoother finish when you're done.
Step 2 - Priming
Once the surface is even you can use a coat of primer over the filler area. A foam brush works best, since you can maneuver it in smaller places, but any small paintbrush will work. Take your time and make sure the primer coat doesn't drip or bleed on to any other part of the surface.
Step 3 - Repaint
With the primer in place and dried, you can start to paint over the area. If you take your time and pay attention to what you are doing, the new paint will blend in seamlessly with the coat that is already in place. You may still see a small amount of the filler while the paint is wet, but as it dries it should become less noticeable.