Trying to paint 8-foot high plain doors i.e. no raised panels but still have a lot of brush marks after it is dry. After I sanded the old surface, I undercoated with Behr 75 Enamel Undercoater Primer and Sealer and then used a Prudy 3" brush, Behr semi-gloss paint and Floetrol Latex Paint Conditioner. I applied the paint in a vertical direction i.e. parallel with the longer side of the door. Any ideas how to eliminate or at least minimize the brush marks. Should I use more conditioner? Bigger brush? Any help would be greatly appreciated since I have 12 more of these to do.

I've painted my hallway only to discover patches of bubbles coming through. l washed the walls beforehand with tsp, rinsed, filled cracks and holes with drywall compound, let dry for over a week, then sanded. l used a latex primer/sealer first, let that dry for 3 days then painted with latex velvet. l've had so many conflicting reports from hardware stores, can anyone out there come up with answer to cure this problem. Merci.