Haven't done much of this in years, but we used to THIN furniture glue (like Elmer's)down to just about water, brush it on, coat it with wheat husks, let dry, then coat with glue again.
Now days the Kitty Litter idea would sound better, or some other rough substance.
The glue not only acted as the adhesive, but gave the decoy a little more water proof coating. The final glue coat would sort of knock down the wheat husks and give us a little smoother surface if we decided to do any detail at all. We did not get fancy with ANY PAINT JOB, just put a pattern paint job on them.