is it a dark or light grey? is the blue royal, navy, light blue?
it depends. If its a standard grey and with a royal blue ink I would not underbase. you can use normal plastisol ink for this job. you can also cure as normal. just dont let your dryer get TO hot.
I've been extremely scared of printing 100% polyester. 50/50 cotton/poly blends are difficult enough with my poor dryer thanks to dye migration issues and lack of a consistent temp.
I believe special inks are required for this application that cure at a lower temperature, so that the polyester doesn't shrink. (causing registration issues on multi-colored jobs or actually deforming the garment.)
you should be able to print as normal (no underbase) with medium grey and a navy blue.
otherwise all you would need is a low bleed white ink as an underbase if you were printing say, yellow on a navy shirt, and you shouldnt have any migration problems. Other than the low bleed white, no special inks are needed, cure as usual at 320.
Yep... rule of thumb, if the ink is darker than the shirt, just do things the way you normally would (unless the polyester garment has a sublimated pattern on it... different rules apply to those.) If you need to print white on a darker color, use a low-bleed ink.
Screen-printing on 100% Polyester? 
