Ok then....remove all the fittings first....but keep the spindle in your pocket in case the door slams shut while your in the room.
A 'good thrashing'...it certainly will as paint doesnt take at all well to varnish.
After sanding the first thing you will need to do is get some knoting over the knots to prevent bleeding.
then give the whole door a coat of some sort of melamine primer...id suggest Dulux super grip......i use it all the time..its great for problem surfaces
Next.....you need to give the door a good sanding down, this is crutial, and you must do it in between each coat
Once sanded and dusted off then give the door a good coat of undercoat.....and sand again with wet and dry paper to remove the brush marks...
Then i would suggest two coats of gloss...again, after allowing the first coat to dry....lightly sand with wet and dry paper...fine grade...(400)
The reason for two coats of gloss.....the smoother the surfaces, the higher the light reflectance will be....the shinier the finish.
The darker the colour......the deeper the shine...
Dont use one coat gloss's they do not have such a deep sheen.
Invest in a good quality brush.....such as a hamilton perfection...expensive but worth it in the long run.......certainly avoid the '25 brushes for a fiver' jobs.....the card they are fixed to is worth more than the brushes.!
Tip:- when using wet an dry....leave it to soak for five mins in a bucket of warm water with a dash of washing up liquid first.
The washing up liquid in the water will also help the water stay on the surface of the door when you wet it prior to rubbing down......keep it rinsed and dont allow the abrasive paper to clog.
Wear gloves.....wet and dry paper cuts through your skin very easily and you wont notice it until its too late....and you will have taken a few layers of skin of!