Water collecting between bottom rail and cill

[Mike MacDonagh]

I had a rotten window replaced less than a year ago. The hardwood replacement has two opening casements and was installed having been primed. I painted the window myself (2 coats of decent acrylic paint) but recently found that both the paint and primer was peeling off the front portion underneath the bottom rail (as far as the rebate) and up the front of it and the revealed wood was very wet. There was also some paint coming off the cill, directly under the bottom rail. I have allowed it to dry out and put a good primer on but, before continuing, I'm trying to work out why this happened.
One casement was bad all the way along but the other was just bad at one end. One guess is that the gap between the outer edge of the bottom rail and cill is too tight and so the water gets trapped there rather than draining away. That gap seems slightly wider in the unaffected section of the second casement (we're probably talking about 2mm as opposed to 1mm). Does this make sense as a cause and would it help to take another 1mm off the bottom of the front portion of the bottom rails?
The alternative cause would simply be that it was a less than adequate paint job and I just need to make sure to do a better job this time!
I'd appreciate some thoughts from people more expert in this area than I am.

 

[DiyNutJob]

Anywhere where water lingers/pools/hangs, there is high risk of damage. If the wood is allowed to get wet, then the paint fails. I am unable to translate your words into an image. So, I don't know what you are talking about. Not sure what is a decent acrylic paint. For exterior wood I suggest 10 year oil paint on good primer. The cracks/openings/paint-edges must be sealed or water will enter from there. For difficult to access locations, I go 2 primers, and 3 top coats. For easy access locations, I am less stringent on the coats because I can always fix it later if there is a problem.