What looks worse than a dirty door?

How about a dirty door that you paint bright white but don't properly prime so the paint peels off in giant sheets?

Yeah, renters don't like these doors.

 I thought to myself "wouldn't it be nice if these gross faded doors were a nice bright white?"  Well, it probably would be if I hadn't just gone nuts with the white paint and put three layers on.  Of course, the first time the door gets nicked, the paint layers peel off.  Now there is something oddly cathartic about peeling big layers of paint.  Kind of like trying to peel an apple in one long strip.  But after a couple of hours of peeling paint off with my newly manicured fingers and trying to scrape it all out of the lovely molding, the joy of peeling is over.

So, I did the right thing (the second time around).  I used TSP on the door to clean it off, then did soem test patches with 80-grit sandpaper.  I did tests where I sanded slightly, sanded hard, sanded over paint that wouldn't peel off, and paint that had completely peeled off (i.e. the original paint).  Then, I tested each patch with a layer of oil-based primer to see how well it bonded.

Test patches

Turns out, all the patches worked out great, so I went to work on the whole door, lightly sanding to get some grit, and then putting two light coats of the oil based primer (I used KILZ) on top. I thought about putting a couple of coats of white paint on top of that, but the primer is white, the door looks great, and why go too far?  Going too far too fast got me into this mess to begin with.  The doors are back to pretty again!

Yay, pretty white door.  Now if I had time to change that yellow wall color...

Time wasted, lesson learned!  What's a DIY mistake you've learned from?