The bits seem to be: 1. Dust from the air 2. "bits" in the undercoat that show up more in the gloss coat 3. Bits that were trapped in corners of mouldings that you though were gone 4. Bits from re-using a paintbrush 5. Bits possibly already in the paint.
Tips I have heard are: 1. Hoover the room first 2. Hoover the workpiece 3. Clean the workpiece with a tack cloth 4. Use a new brush (but apparenly pros use the same brush for years) 5. Close doors after painting and leave 6. Pick out any bits that you see as you paint with a rag 7. Sand each undercoat. This is the correct way to do it, but it ridiculously time consuming, and a potential source of more dust.
Problems are: 1. The bits sometimes only appear in different light that the conditions during painting, so you only notice it when too late. 2. Once bits appear, they get on the workpiece / brush / paint and seem to multiply. 3. I don't know what finish I am aiming for (nothing is "perfect")
I fitted primed white bannisters recently and they have a lovely smooth finish, but should really be painted to cover joints, edges etc. I do not want to paint these as they will look worse. I do wonder how good it would be if I got someone in.
Any advice or experience on this issue would be very helpful.