Hi, do it in sections, ie a square at a time or if it is a panel type then do the panels first then the edges. Are you using a undercoat with it or just a one coat gloss finish. Paint a section at a time and lay off as you go all in the same direction. There should be no need to de-ripple as you call it when you have finished. Doing this will only make it worse as you have noticed as by the time you get to the end the paint will have already started dry. It is possible that the paint that was already on the door and the new paint are not compatible but this doesn't happen often. One other thing is to make sure that you prepare it well and make sure all dirt and grease are off before you start to paint.