This usually happens when using paints with a different solvent base say an enamel (turps) painted over using a paint that uses a different solvent base such as an acrylic duco, this will cause the frying of the paint such as you have experienced it can be overcome by using an undercoat before applying the new paint, sorry I cannot tell you which is the best to use but your local paint shop should be able to advise you.
Regards Beech.
