What works for me:

1. Scrape off all loose/cracked/chipped areas thoroughly. As
necessary, strip areas.
2. Fill cracks with caulk compatible with paint and exposure.
3. Sand everything smooth.
4. Prime all exposed areas; alkyd may work better here.
5. Paint, 2 light/moderate coats, at dry, moderately warm time of day
(differs by side of house, even).

Emphasis on prep, for adhesion of paint.

HTH,
John