Testing if the existing paint on fence is water based or enamel.

Apply methylated spirits to a rag which is a different colour to the paint. eg white paint = dark rag. 

press, and hold on the painted surface, and then rub. 

If the paint color comes onto the rag it is water based, and can be painted over using StudX or StudX Full Body.

If the paint does not come off, you will need to prime the fence.