I have done quite a bit of glassing with poly,if your using the oil based version just wait for it to cure for a couple of days, so when you sand it there is a powder,if it balls up then its not ready,and needs a little more time to fully dry.
then just cover it like you would any other surface.
if its water based poly then you will need even less time for it to dry to the touch in a couple of hours,over night to sand it.
word of caution when covering over any of the poly types go easy on the heat,it will soften the material and lift the glass if you get it too hot,just enough to apply the covering material,whatever you use.
most of the time I have painted directly over the glassed surface,I did cover the floats I built for my Nextar with iron on film they came out pretty good,alot stronger than if I hadn''t for sure, I am definatly a fan of using water based poly for glassing models for paint it provides a nice base surface and is alot easyer to use than any other products I have used for this purpose.
some examples of WB poly glassed surfaces,floats are covered in ultra-coat,the Spitfire,and P-47 are both painted,on the P-47 I used all water based products glass to finish.