Make sure you are using a poly white when printing to control the dye migration from the polyester. You can build up the emulsion in the screen creating a deeper well for the ink to help print white in one stroke.
If you would like to print an extremely thick coat of ink (on a jersey for instance) you may consider using an 86 or even a 60 mesh. For most prints, a 110 mesh will work great! One of the main tricks to get great in coverage on a garment is stencil thickness. The thickness of your emulsion directly dictates the thickness of ink laid onto the garment. Picture your screen stencil as a well, it only holds as much ink as the well is deep. With one pass, the maximum ink coverage you will achieve will be the thickness of your emulsion. Coating the screen multiple times on the shirt side or outside of the screen will create a thicker stencil and thus yield more ink and better coverage. You must buildup your emulsion on the outside of the screen, if you coat the inside of the screen, then you'll have to push through the stencil and work against yourself. To make an optimal screen for white, coat the screen once on the outside and once on the inside, let dry, coat again on the outside, let dry, and coat one final time on the outside. Also it is best if you use the round end of your scoop coater to lay more emulsion down. Keep in mind that since you are using a thicker emulsion, you'll have to increase your exposure time in order to thoroughly expose the screen. We recommend using a dual cure emulsion for the easiest results and post hardening the screen after washout.
Here is a few videos
The Screen Printing Process, Printing White Ink
printing white on polyester and spandex 
