It's amazing to me how often contractors and homeowners alike apply latex paint directly over a semigloss oil finish without first roughening up the old surface or applying a primer. Before long, the slightest ding can result in a large chip, exposing the old oil-based finish.
I've owned two homes with this problem, and I've spent hours sanding off the latex layer on more doors than I care to remember. But recently, I stumbled upon a great stripping aid: water. I've found that if the latex surface is dampened with water a few minutes prior to sanding, the top coat will easily lift off the oil-based undercoat with gentle prodding from some 80-grit sandpaper. A wet rag is all you need. Just make sure the paint stays wet for a few minutes prior to sanding. I've often had entire sections peel away from a door.
Once you've removed the latex, rough up the base coat with some sandpaper and give it a coat of primer. Now you're ready for a new top coat.
Jonathan Applebaum, Westfield, NJ
From Fine Homebuilding 137, pp. 34
March 1, 2001