Jesse-

I would use an oil based primer. The oil primer is good for sealing out oils from the wood so they don’t bleed through the paint. The oil will also soak a little deeper into the wood and provider better adhesion to the substrate. For the top coat I would use Duration paint. It’s a little spendy but you’re not going to use that much paint on the railings and you want something that is going to last. Railings take a long time to prep and paint so don’t go with a cheap paint.