How do you paint over an already painted wall?

I'm trying to do a painting of a fairy for my step mom. I've used both acrylic and oil paints on it and it just won't apply and blend correctly. Do I have to do anything before hand to the area I want to paint? Please help me. Thank you.How do you paint over an already painted wall?
The quickest/easiest/cheapest solution is a cleaner from Mr. Clean called the Magic Eraser. It will clean most anything and painters use it to ';de-gloss'; walls so paint will adhere better. While you may need to prime your wall, I'd try scrubbing the area you want to paint with a Magic Eraser first and see if it solves the problem.How do you paint over an already painted wall?
If you're using acrylics and oils at the same time, that's probably the problem. Acrylics and oils don't work together. Also, trying to paint acrylics over oils is an exercise in masochism.





You'll need to strip off the work you've already done, probably with mineral spirits or something similar. Make sure the surface you plan to paint is clean and dry, with no cracks or bubbled paint from previous paint jobs. If you plan to paint with oils, then I would recommend gessoing the area you want to paint. Gesso is used on canvas by artists to seal the canvas and provide a smoother work surface, and it's available at most art supply stores. If you're using acrylics, then a basic matte or semi-matte primer might be helpful, but I'm not sure it's truly necessary.
prime it first?