I always bump in these threads - I've tried a lot of things for sealing foam.
The only product that works for me (and for a lot of other people) is Flexbond (it was mentionned by Joee - but nobody seemed to notice). It's a product from the company Rosco - if you're interested, look up the distributors in your region.
It's pricy, but it's so much better than Mod Podge or Gesso.
The only thing I can't comment on is how it compares to Plasti-Dip (it's not easily available here - so I've never tried it)... but some people here are swearing by this product and what it can do for foam build.
So, if you're looking for a durable (I mean it will last for years, through many wearing if taken care of properly), flexible sealer for foam that will also act as an adhesion promoter for your paint job - go with Flexbond, you won't be disappointed.