People constantly are making their own screens and using paint, blackout cloth, and other options to make their screens. Paint can be very good, but you need to be aware of having a very flat surface for painting.
Goo Systems sells DIY painting kits that have very good reviews. But, also people use different paint combinations that they try out for the best results. Try the Screens DIY subforum over at AVS Forum for some great information on painting screens. Many people go up to 120" diagonal with painted screens.
After seeing a DIY screen compared to a high dollar screen I would NEVER buy a screen again. Unless I needed a lot of gain for some reason.