Painting exterior railing
Think of oil and latex paint for what they are.Oil soaks in...latex covers the surface.Latex can go over oil,but oil cannot go over latex.Your oil primer is fine on bare spots,however,if the previous paint was latex,then your finish coat should be the same.To make matters simple,you could have primed and finished in latex.
I would recommend oil primer because is grabs better on wood (in my opinion). As for the topcoat - a good quality 100% Acrylic latex will outlast any oil outside.
So - this should be - 1 coat of exterior oil primer - and 2 coats of 100% Acrylic Latex topcpoat - either Semi-Gloss or Gloss
If you use latex primer you had better use a latex top coat too.
If you use a oil primer use oil or latex top coat.Some people say this does not matter,
Everyone has their own thinking and opinions even the paint makers.
I have been doing this for awhile and their are a few old schoolers out their that say this but we have never seen this,so no matter what you should be pretty safe.