I thought about this a bit more because I remembered asking my buddy at
his custom body shop the same question. I watched him mix a small
amount of paint and thinner in a little bottle and then into the AB cup
for some touchup work.
Beside the answers you already received he also mentioned this -
"Whatever I put in the cup first, will have the tendency to start
dripping down into the AB body cavity. So how will I get a properly
consistently mixed paint/thinner ratio when there is a small glob of
paint or thinner already loaded waiting to be sprayed? Especially if I
have to mix a little more? Doesn't the words gravity-feed mean
anything? It doesn't take the air pressure to cause it to start,
gravity is all around us."
Ok, so I felt stupid!
I was only used to painting with suction-feed, what do I know.
I have always used the same technique when painting cars, mix paint in
a separate container, pour through filter into paint cup. Same thing
with my bottom feed AB. Only makes sence to do it for a top feed also.
Easier to mix paint/thinner ratio, at least for me, so it will be more consistant.
Can mix a larger amount as needed ahead of time.
Thinned paint is easier/faster to flow through a filter than straight paint.
So it's one extra step and an extra bottle to clean.
I think it's worth it, but what do I know.