The product that you are referring to is called "Magnetic Creations." Although it goes on like paint (with a roller), it is really an undercoating that is designed to be covered with a finish coat of paint.

The product is filled with tiny iron particles that attract magnets, thus eliminating the need for tacks, pins and tape when hanging posters, pictures and plaques. You can get more information on the product by calling (800) 850-0113.