Reply by bilyo

Based on my experience, a good quality exterior stain will usually hold up well on vertical surfaces. Where they have problems are on horizontal surfaces. So, IMO, apply an exterior opaque or semi-transparent stain. If I understand correctly, some require stripping before re-applying, but there are others that don’t.
Since you are using pine, my first concern would be that the frames are constructed so as not to trap moisture anywhere. Otherwise, they will deteriorate quickly. You might consider using pressure treated pine.