If they don't look heavily discolored, try Smart prime ( Zinnser 123 Plus- same thing). It is a waterborne alkyd:huh:. Just let it dry good - overnight- before finishing. Does a very decent job on many stains. But the oil would be less of a risk, more odor.
Come on Chris- there are so many -
Advance, Zar Ultra Max- SW WB Alkyd, etc. I believe they are a soy based oil- so have been made to be carried in a water vehicle.
Ok great, did I Mention that it looks like they were painted already? sorry , does that change what type of paint to use?