Hi Chris The act of scraping will only remove lose paint: It doesn't clean (molds, dirt, chalking, etc) nor does it remove old oil based paint (checked and cracked).
If the topcoat of old paint is still adhered well and is not cracked, then a pressure wash followed by scraping and priming of bare spots will give you a resonably good job. However, unless you taper the edges of the paint around bare spots, you will clearly see these edges under your new coats of paint.
Personally, I now take the time and remove all the old paint. If you do this then clean and sand the clabbords and prime before putting on at least 2 finish coats, your results will be very impressive!