You definately do not want to use a flat or satin finish on a fence or on
trim. They are both porous and stains will penetrate the full thickness of
the paint film and you will be repainting constantly. Use a semi-gloss
finish or gloss.

To make things easiest for you in the long run you should do a coat of oil
base PRIMER that is tinted a shade lighter than your top coat paint. You
can then use a quality paint such as Sherwin-Williams Superpaint latex
(water-based) for your TWO top coats. Don't worry, the latex will stick to
the oil based primer but it will not stick to oil based paint. I highly
recommend going to Sherwin-Williams and telling the store manager what you
intend to do and you will receive some good advice.

If you wish to prime with a latex primer then I highly recommend Zinnser
1-2-3.

Take take any shortcuts. At least 1 primer coate (2 is better) and always 2
top coats. Pressure wash or at least do a bleach wash of the fence prior to
painting.

Bobby Baxter
Happy Moose Gardens
http://happymoosegardens.com