Bi-fold with louvres? I don't see any reason to prime them. Already painted with a good oil paint? Sanding will be necessary either way. Just sand them and use a good top coat. Louvres are such a pain to begin with, why go through the extra step if you don't have to?
Because making the switch from oil to latex requires at the very least a good latex bonding primer. Sanding isn't enough. I personally would recommend oil paint or an oil primer and then latex paint.
Latex Primer: Good for Drywall, that's about it.
Behr Paint: The plastic buckets are good, that's about it.
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