How to paint over oil base paint with latex paint?

To paint over oil based paint with latex paint you need to sand the surface to remove any gloss from the previous coating. You then need to undercoat with a waterbased undercoat or use a bonding coat in New Zealand we use primerlock (waterbased) Most paint retailers will have a product similar. Explain what you want to paint and they will be able to help. Once you have done the above you can then apply the latex paint.Some people will say that you can paint straight over with latex but the problem then is once it is dry you can scratch the paint off with your fingernails.