Most resins have enough flex that they will not crack when bent. Coating craft foam in resin improves tear strength and helps to seal the surface off so it can be painted. Polyester resin, particularly when mixed with too much catalyst, can be a little brittle. But provided you aren't too hard on your armor, this shouldn't be any problem.
One issue is that the result isn't really all that much more effective than just using a couple coats of white glue (PVA glue) + a touch of water. Doing this is much easier since it can be done inside without any special safety equipment; cheaper too.
I've read that it is difficult to get an even coat when applying plasti-dip via brush, but I still haven't gotten around to playing with the stuff myself.