Q. Patio chairs that I sandblasted, zinc-primed and powder coated about one year ago have started to rust in the area where the expanded metal seat fits into the formed frame. What do you suggest to fix this problem and what do you think went wrong in the original process? I thought that I had sprayed into the tight areas well, but evidently this did not work.—K.T.
A. The rust that shows in the pictures you provided is located in an area that would be hard to get completely clean. It is hard to be certain about the failure mode looking at just two blurry pictures, however, the location suggests that there was some corrosive foreign material left inside the affected area before you coated the parts. It would not take much dirt, old paint, rust or other foreign material to start a corrosion cell in a tight area like that. If a corrosion cell was present before you coated, your good work was wasted. Rust never sleeps.
You will need to remove all of the old coating, of course, and that area of rust will need some special attention. A thorough blast might get it all out, but I think you may need an acid solution or something to be sure you get down to bare metal and get rid of any contamination. Try:
- Strip the old coating.
- If practical, acid pickle the chairs. If you cannot immerse them, apply the acid solution by brush. Let it soak long enough to break down any remaining coating or other material.
- Thoroughly rinse with demineralized water; make sure the acid and all foreign material is gone.
- Blast to white metal.
- Apply your zinc-containing powder.
- Apply two layers of topcoat. (Be sure to test inter-coat adhesion.)
That should take care of it, and you will have long use of the chair without rust.