The trick with poly feeders is they need to be painted inside - 3 or 4 coats is needed. If you can get it in small tins acrylic gloss is ideal, Jonsons Paints do one otherwise any gloss suitable for outdoors use will do. The bit the bees walk down needs to be rough. My suggestion is to sprinkle coarse salt on the wet paint and then wash it off when everything is dry.
The poly has to be painted otherwise the syrup will soak into the body of the feeder as contrary to what you might think the poly is not totally waterproof and when syrup sits in the feeder it leaches into the body of the feeder, which slowly gets heavier. It is not a problem externally, rain water won't soak into the hive but syrup in a feeder will.
The original Canadian poly nucs were often used by squirting sugar syrup through the entrance where it sat in the bottom of the nuc and could be accessed by the bees. But for the reason given above the inside of the nuc needed to be painted.