What's the best way to paint over dark varnish?

I have dark varnished internal window frames and door in my kitchen and I'd really like to paint them white to match all the rest of my windows. I've read that I need to sand down the varnish, then use Aluminium Wood primer, then undercoat and then gloss. Is this the correct method? How much sanding is necessary?What's the best way to paint over dark varnish?
It is not good to paint surfaces that have been varnished. The varnish is rigid and should have soaked into the wood sealing it from wood primers getting a good grip and making paint liable to chipping off. Generally it is not enough to rub down or just matt the surface. Even stripping doesn't solve the problem.





Almunium primer is for sealing particularly resinous wood, so if you insisted on painting stained (rather than varnished) wood you might use it also.What's the best way to paint over dark varnish?
Cleaning the old varnish is extremely important. Clean the surface well using Krud Kutter or something similar.





If you are sure that the old stained/varnish surface does not have lead (banned in U.S. in 1978), then you can use a scouring pad in conjunction with the cleaning solution which will serve to help clean and degloss at the same time. Don't use the scouring pad if lead is present (see EPA lead page for lead-safe practices).





A good oil based ';bonding primer'; (like Zinsser's Coverstain or XIM 400 white or something similar), will adhere to the varnish and serve the purpose of anchoring the subsequent finish coat of paint to the surface.
Sand it to strip it, go with a good primer, and the undercoat and a gloss are good.





You can sand until you don't see the dark color if you want. Or just take off the top layer so you can apply the primer easier.
Just enough to scuff it up and take the shine off so that the paint will stick.Use a good quality oil based primer. and finish with a semi gloss latex paint.
you will have to use a very strong stain blocker/primer or the staining will bleed thru