i use a 3.0 tip for spraying the standox poly, and i use a iwata/air gunsa gravity feed poly gun, the klc is nice for urethane primer but too nice for poly, the air gunsa/iwata poly gun can be had for 100 bucks online and it works great, i only mix 1/2 quart at a time when spraying poly as it sets up too damn fast, and it is a mess to cleanup if it sets up in the gun, don't ask how i know.
at minimum i would use a 2.5 tip with the standox, standox's pe spray filler is around 50 bucks a quart with hardner, 6-8 will do an entire vehicle with 4 coats.