To answer your question effectively, it would really depend on the existing paint. The industry standard is to convert to water-based (Latex) as it is a more environmentally friendly product. Therefore, if you currently have an oil-based paint, the best method would be to first apply a fresh coat of oil-based primer and then a coat of water-based. This will give you the best outcome and convert the existing to the recommended water-based (Latex). The reason for the first coat of oil-based primer is because the new water-based primer will not adhere to the matured oil-based paint. It will begin to peel once dry, but it will adhere perfectly to the fresh coat. However if you currently have water-based (Latex) it is recommended to use a water-based (Latex) primer.
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Water based with acrylic binder paint is best recommended for exterior applications while oil based work better on metals or bleeding woods