Rusting Through the Paint

Question:

We manufacture large computer enclosures. The problem we have is that they were stored in a damp warehouse before shipping. The bottom panels made from 16-gauge CRS exhibited rusting through the paint. We had some issues with the paint manufacturer and they assured us the problem paint is eliminated. Now, the real question is—if it is surface rust showing through the paint, do we have to strip and sand the bottom panels and repaint or can we lightly clean the surface removing exposed rust and repaint? In other words, I don't know enough about the properties of rust. HELP! G.D.

Answer:

I could never recommend painting over rust. If the rust is coming through the paint, the entire surface may be corroded. What this means to your customer is, he could get rust on his fingers every time he touches the cold-rolled steel (CRS) bottom panel. Not a good thing. It sounds to me like you painted directly over the CRS without pretreatment, which is another bad thing, especially if you have to store the product in a damp warehouse.

There are still a number of questions to be answered. Was the paint film too thin? Was the paint applied the right one for application on CRS (for example, was it a latex paint)? What kind of pretreatment was used? No matter what the answers are, to solve the immediate problem, I recommend you strip the paint, clean off the rust, apply a pretreatment and then repaint the affected CRS panels.