How do you manage to get on even coats of plasitidip because when I spray it on it either splays in blotchy messy sprays, or completely over coats it
Blotches might also be a procedure issue and not just the can being not warmed up. When you start/stop spraying you don't want to be above the piece, you want to be past onto your dropsheet/newspaper/whatevers so that you're getting PlastiDip the whole time and not old goop that's been trapped in the nozzle, accelerant only or other stuff that'll cause you grief. Even if you hold the can upside down and spray to clean the nozzle after each use, just assume there's gunk in there that'll give you a bad time when you pick up that rattle can again.
How to you get the foam to not fall apart after painting
Are you using EPS or XPS foam? This sounds like an aerosol eating polystyrene type question because I've never had an issue with EVA falling apart. If that's the case, precoat with a non-aerosol brush on acrylic gesso or PVA coat so that there's a layer to protect the foam.
If it's not EPS or XPS you're using... What glue and paint are you using that's eating one another?
How do you get the foam to not be so bendable
Building in layers and shapes that support each other. I have two techniques that I use fairly regularly, one is backed in science and construction, one is cheat-y and I only use it for very thin pieces that overhang.
For structural pieces look to ship building and the concept of ribs and stringers. Add in cross members to strengthen against rotation and flex.
For overhang I use sheet aluminum cut into strips layered between the main foam and a sheet of 2mm foam. This can be bent and will hold it's shape as well as pull the foam into that shape if the curve you need is beyond what heat forming will keep.
How do you paint/coat the foam in nice even sprays?
Heat seal, thin coat of PlastiDip/FlexiDip, three more coats of rubber coat, primer coat, two or three colour coats. All of them have wait times in between that are laid out on the can instructions. Patience is usually the trick to a good paint job.