I agree that a high quality exterior oil based primer would be best and the best topcoat would be 100 percent acrylic. Sprayers are actually more designed for oil based coatings than latex coatings. The best thing to run through a sprayer is actually lacquer. Ever since latex/acrylic coatings became the main coating we run through our sprayers, our maintenance on our sprayers have been more costly. Also, for exterior wood priming, spraying may not be the best thing. The best protection you can give to wood comes from penetration of the coating, which can really only be achieved by pushing the product into the wood pores with a brush or roller. You can actually use the sprayer if you wish to get the material on faster and then backbrush it. I would opt to backbrush or backroll the topcoats as well. The absolute best thing you can do is backprime the wood as well, prior to installation (prime all sides). Cut your boards, and prime the end grains too...the part of the wood that gets buried in the ground. That is usually where moisture enters and the wood will start degrading there and move upwards...anything you can do to slow that process will help the life of your paintjob. As a rule of thumb, I always tell people that wood sitting on grass, in the dirt, etc. is really void of any warranty that I could offer or the manufacturer of the paint could reasonably stand behind. Wood sitting in rain, snow or mud just isn't going to hold up long term.

The latex paints today are very durable and should hold up fine. If you want to use oil paint on the door, you can. If the paint is peeling, you should remove all old paint that is flaking and then sand the door down, prime with a good primer(red oxide primer if there is any rust) and then you can topcoat the primer with your choice of paint. If you use oil, you might want to consider using One Shot Lettering Enamel by Spraylat(available at paint stores and sign shops). This is a very good quality paint that professional sign makers use in their work. One word of caution, some colors still contain lead. Be sure to ask before purchasing and using the product.

I agree with paintguy. Acrylics increase their adhesion with time. What scraps off after 2 hours may well be adhered after 2 days. As to Behr paints, I have personally made test samples of Behr Ultra, Marquee and WB Alkyd over sections of hard, slick, high gloss oil paint. I hand these to customers and defy them to scratch it off with their fingernails. They are wearing thin with literally hundreds of tries, but they are still intact! Wanting to see the worst case scenario, I did not sand or de-gloss the oil paint. I am an old school painter, raised on oil pains and somewhat of a doubting Thomas. I have become a believer. If someone's paint is "balling up", I would wonder what is on that surface? Was it cleaned of oils from hands or other pollutants?

I don't think it is worth it to strip the paint in your entire house just so that you can convert back to oil. Stripping paint is a very labor intensive and messy job. Paint will chip off in high traffic areas with latex and oil based paints. You should remove what is loose/not ahdered by scraping and sanding and then repaint with a 100 percent acrylic paint. Many DIY painters do not sand well enough and that is why the paint may not be adhered in some areas. You really want every inch of the trim to be very dull and clean before applying any paint to it.