I am currently in the process of prepping an outdoor wood railing to re-paint it. I started the project since the old paint was flaking off in spots and on some railings, the wood had rotted through (I am replacing that wood). I have been stripping the old paint where it was flaking and then sanding down the entire railing. I've been told to apply an oil based primer where the wood is now bare and then apply an outdoor paint (not sure whether latex or oil - what's best?). I am new to this, but does it matter what type of paint the old paint was - i.e. if I am only putting primer on the bare wood, will that make a difference when I apply the top coat of paint? And how would I test to know what kind of paint it is? I just want to finish off the railing so that it looks good and so that I won't have to re-visit it too often. Any help or insights would be great. Thanks.

Painting exterior railing