Styrofoam is used in many craft projects, ranging from costumes and plant displays to sculptures and theatre sets. If you plan to paint your styrofoam with regular paint, you will need to learn how to seal it properly first. If you apply regular paint to styrofoam without sealing it, it will eat away at the foam and cause it to melt. Properly sealing the styrofoam first prevents this problem and gives you a relatively smooth surface to paint on.
Cover your work surface with a plastic tablecloth. Plaster of Paris is very dusty and tends to leave a film on surfaces where it is used. The tablecloth will protect your table or countertop from this film.
Mix two parts plaster of Paris to one part water in a large bowl, being careful to remove all lumps. Lumps will affect bonding and give you an uneven surface on your styrofoam.
Use your large paintbrush to apply plaster of Paris over the entirety of the styrofoam. Allow the plaster of Paris to dry and then repeat the process until you have three coats.