A.Henri de Marne responds: First, you should find out what caused the steel saddles to stain the beams. Such stains are usually caused by water contact.

Since the steel saddles would not have stained the beams if they had been painted with a rust-inhibiting paint before installation, I assume the saddles were installed raw. You can use oxalic acid to remove the stains, but the work will be tricky because the acid solution should not come in contact with metal of any kind. You will have to mask the metal carefully before applying the acid solution to the beams.

You can buy oxalic acid crystals from a paint, hardware, or janitorial supply store. You won’t need much. Dissolve the acid crystals in warm water in a glass or plastic container. The acid crystals should be mixed to saturation — that is, until some of the crystals will no longer dissolve.

Carefully apply the solution to the stained area with a small brush, without letting any drip. Removing the stains may require more than one application. Use extreme caution; oxalic acid is a very potent chemical. Wear rubber gloves, and skin and eye protection.

Once the stains are gone, wipe the treated areas with a sponge or clean cloth dampened with white vinegar in order to remove any crystal residue.