Using primer and undercoat on sanded down window frame

[woodman-46]

Hi, sorry if this has been asked before. I am renovating some wooden window frames and have sanded down any flaky or damaged paint. Most of the existing paint is in good stable condition and I am going to leave that as is but where I have sanded it is down it is to the bare wood in places. Obviously I am going to prime and undercoat the bare wood but do I only do this to the bare wood or do I continue over the existing painted areas as well , at least onto some of it around the bare wood.
Any advice welcome.
Thanks

 

[opps]

You spot prime- apply the primer to the bare spots. Then I would spot undercoat. Lightly sand the UC to flatten it and apply UC to the whole window before glossing it.

 

[woodman-46]

Many thanks for your advice!