Here's my take...
Painting: make sure you are using a spray paint that is foam safe. If in doubt, don't paint with it.
Decals: Draw it up or "borrow" it from the web. Print on photo grade paper and tape over with clear packaging tape.
Film covering: If you are building a foamy, use laminate. Its that stuff used in laminating machines. Set the iron to wool unless it is worn out.
Be creative and make it what you want it!