Painting woodstained window frames

I want to paint some woodstained exterior window frames with white gloss and was wondering on the best preparation. Some of the frames are a bit flaking but others are quite sound. I was going to give them a good rub down and scape off any flaking stain and prime any bare areas, but was wondering what to put on (if anything) the sound areas before the undercoat and gloss.

Would the stain show through if I undercoated and glossed directly or should I use some sort of primer? Would aluminium primer be a good idea or something like Zinsser BIN?

As far as I know it was a water-based stain that was used.

Thanks in advance.

Your right that you def need a primer and ali wood primer is fine along with a host of zinsser products

Thanks for the help.