Wood – Skirting board filler chipping off

Priming the doors will do the most to ensuring your previously stained doors have a consistent, even finish.

I also like to lay a door flat and paint with a roller - makes the job a lot easier because you're not having to mask off each doorway, worry about runs and drips, etc to paint the door while it's hanging. We didn't have enough space to set each door aside while it dried so we had to wait for each side to dry before we flipped the door over (and was only able to do one at a time), but they turned out really nicely so the extra time spent waiting was worthwhile.

Yes this can happen, especially if you don't remove the masking tape straight away.

It might indicate that the paint isn't properly adhered to the underlying surface, but it can happen even if you've prepared the surface properly (cleaned and dried).

The only real solution is to use something else - a straight edge held to the wall is always a good choice. However, that might not work well if the wall is uneven.

You could draw a line where you want to paint to and then hold a short piece of card up to this line as you paint in small sections.

From your update it sounds like when the previous paint was applied the surface wasn't prepared properly. Your best bet in those circumstances is to remove the top layers of paint - or even all of it - until you get a sound surface for you to paint on.