Just a heads up man, necroposting (posting on a thread that is a few months old) is frowned upon, as it bumps down newer subject matter.
I see this is your first post, so welcome to the community! Please do some research before posting questions, as there is a ton of awesome info on this site, if you just take the time to read and find it!
Plasti dip is most commonly used in the costuming community to coat/seal eva and craft foam before spray painting it. It goes on thicker than regular spray paint, but not super thick. Cardboard is a very hard medium to work with, as it's very hard to get rid of the corrugation lines and making the end seams clean. Aside from using some form of resin and body filler, it's near impossible to do. I'd recommend enjoying your first prop as it is, and take what you learned while building it to move on to more suitable mediums in future projects!