Ok, I admit it, I have bitten off more than I can chew. I had great
visions of sanding back to bare wood my stained stair bannister, but
after much sweat and tears I have decided to paint the damn thing white
instead.

Thing is I am now left with a a banister which is bare wood in some
parts and pretty much still stained in others

Can I just go over the whole thing with my Dulux 'Primer & Undercoat -
for Wood' and then apply 1/2 coats of Quick Dry Satinwood (also Dulux)
or do I need to complete the job?

Thanks.