You must purchase several tools and materials to paint a steel door. To begin, buy is an oil-based metal primer that inhibits rust. You will also need to purchase enamel paint for the door. Both of these products combined should cost somewhere between $20 and $50. You may also need to purchase sandpaper and a block to remove the existing paint on the door. This will usually cost less than $5. You will also need to purchase a paintbrush which will generally be less than $10. This brings the total cost up to somewhere between $35 and $65.