Water & wood glue, or thinned drywall compound, to fill the micro voids of the product. Automotive body filler works too but it's harder to rework. Then sand polish it to as nice of a finish as you can. Apply an enamel and polish it to a mirror finish with polishing compound and a buffer.
If you need a perfect finish you'll still have to polish the polycarb after. Look into chemical polishing. Some good info here: