Hi
You can take a look to this topic:
@djwaterman use it I think, there’s some discussion in topic about obtained results and differences with other tools.
Also the plugin was made before halftoning filter has been rewritten by @Deif_Lou
This post can help you to use the halftoning filter:
But in all case, I think you have to work with flat color and then, apply a filter.
Trying to paint with the effect directly might be hell 
Grum999
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