I made a new box for my 52 Chevy pickup 3 years ago. It was made with new 16 gauge cold rolled steel. I used Omni epoxy primer to seal it up. The metal was clean and degreased before priming. I put the box in the corner of my shop for storage as I worked on the rest of the truck. Yesterday I looked more closely at the box and noticed little rust spots. There are about 20 small rust spots in random areas. There is one cluster of about 20 more rust spots. The cluster is about 5 inches in diameter. Each rust spot has a small area about 1/16 to 1/8 inch in diameter that the primer has lifted. The paint around the rust spot is stained with rust. The rust stain can be wiped off leaving the small spot of rusted metal. Here are some pictures. The one picture is the box right after it was primed. Any ideas what happened? I am thinking I must have had moisture in the compressed air. Do I strip it all off or just spot sand and re-prime. I am thinking that 3 years is enough time for issues to show up and I can just spot sand the troubled spots.