how to "fight" capillary action while brush-painting?

Adding to what other people have said:

1.) Stir your paints, I got a little battery operated paint stirrer for that.
2.) Add a retarted or similar levelling agent to your paint, those will slow the drying time and aso help the colour achieve a more smooth finish.
3.) White & Yellow are best applied in multiple THIN layers with the brushstrokes varying in direction between layers.
4.) using a very fine sanding medium between layers will also help get a smoother finish.
5.) Beware of old paint.

Hope this helps.

Thanks to all of you. First off, I forgot to mention that I am using flat white paint. So, I guess I'll try priming with gray, and if that doesn't work I'll use the dish soap method. I did wash the parts in a mild mixture of detergent and warm water, scrubbing them with a toothbrush. Also, I have in the past tried shaking the bottles, but that leaves paint all around the bottle lip, which dries and makes a mess, and makes it difficult to open the bottles. Anyway, I'll try your suggestions. Thanks again.

I would try this in the future....
1) Prime the parts either with a flat white or gray color coat( I use Color Place really cheap and seems to work fine...Walmart 98cents/can)...
2)Shake the paint well...then open it and stir it making sure you have any of the pigments that might have settled to the bottom mixed up....
Last I would use dish soap, however I would have a spare bottle handy for this not sure about you but I dont want to put dishsoap in a paint jar then forget about it and go to thin it for airbrushing and have more suds then I do paint! LOL
Try the primer first as well as the stir/shake method.
Another thing that could cause this with acrylics especially if the parts are un-primed is be sure to soak all the kit pieces throughly with a mild dish detergant and wash them just as thourougly. This is a very important but often over looked step....Hope some of this helps.

HEY,
I dont know if this is the same for acrylic as it is for enamle, but for enamels when this happens it usually means that all of the pigment is goop on the bottom of the bottle. Hope this helps some.

Randy

You can also try priming these parts first with a light gray then painting the parts with your flat white. Whites and yellows are especially bad in this situation.

Since you're using acrylic paints, you could try adding a bit of dish soap... it should reduce the surface tension of the paint.