DTG doesn't really work on poly. Works best on natural fibres like cotton.
When working with poly, always keep in mind that poly does not like to go over 300 degrees or you will have issues such as dye migration or nice shiny areas. Inks used in DTG printers needs to go over the 300 degrees. We played with it and it does not hold up when you wash it.
How's the washability?I have been doing poly on my FP125 for 2 years now with EXCEPTIONAL success on both lite and dark (white under-base). I use the image armor pretreat and image armor E series ink. I have printed on most of the popular poly shirts, Augusta, Badger and Sport-Tek. Check out the Image armor web site Home - Image Armor DTG Pretreatment Solutions
Have a great day