No halftones, so you (could) use that Poly on the top white as well. For the most part, I don't see many people using Poly on top white. It's not always needed especially for a job like this where you can really use a low mesh on that base that will assist in the blocking, but make sure to also be positive that you are also curing the ink.
The Poly factor and the bleed factor contribute to people under curing these. They feel (too much heat) makes the dyes migrate, so many are prone to back off the heat a little and also the idea of too much heat can scorch or melt these poly shirts. I would make sure that each shirt is truly cured. A cured shirt also helps prevent migration. I've seen orders for 500 and a1000 sweat shirts look to be a cured shirt, go out the door, only to see them in the stores with the whites pink or navy tinged. A little more time and care, to assure a proper block and curing can save tons of money in the end.
It's a fine line. I know, I sound like I know what I'm talk'n about, but I don't really print...so keep looking for more answers and weigh out the best from those who continue or consistently to provide smart answers. I'm not sure if mine is totally correct. It's all based on what I read and remember from the shops I worked at.
This print seems small and not a lot of ink coverage. So I might make sure to use Poly blocker on that top white also. Maybe just not lay it down (as heavy) on the top white.
Also, that purple can tend to be thin. People think to use a higher mesh on that purple so as to not get the ink to stack and make it too heavy. This print is already going to be heavy due to the poly blocker and the mesh required for that. So, I would also use a medium mesh. but coated well, so that it presents a nice clean heavy application of purple ink that will have a good wall. Otherwise, if too thin, it will skate around on that sheet of plastic white underbase.