Don't use a spray paint, it melts the foam. I simply went to the hardware store and either look in the OOPS paint section or just buy a gallon of black latex paint. The paint goes on very easy, and I used an acrylic white paint and a stencil for the cannon's name. This one was obviously stolen from a French Galleon.
Your now authentic pirate cannon can be accessorized by adding a pirate.
This process does make quite a sturdy piece. The cannon below survived being passed around 80 other pirates on a pub crawl.