Do It Yourself: Acid treatment removes rust from concrete

Q. My concrete sidewalk and driveway have large brown rust stains that I haven’t been able to remove. I tried bleach and a chemical remover to no avail. Can you help?

A. Rust stains on concrete seem to respond best to acids of various types. The classic treatment is oxalic acid, which is sold at some paint stores for use as a wood bleach. It is often sold in small bags.

To use oxalic acid, mix one-half cup of the powder in a gallon of warm water and mop it on the stain. Let it work for five to 10 minutes, keeping the stain wet, then scrub with a stiff-bristled brush and rinse.

If you can’t find oxalic acid, a bathroom rust-stain cleaner that might work is Zud, which contains oxalic acid and is sold at most supermarkets. Follow directions on the container for using Zud, a powder, or make a solution and follow the same procedure described above.

When using any strong acid cleaner, wear eye protection, rubber gloves and clothing that will protect your skin. Also read the cautions and directions on the container.

If a stain fades after the first application of a cleaner but is not removed, try again. If you get no results, there are some other possible solutions.

Some people swear by lemon juice, letting it work for five minutes, scrubbing, and following with a similar treatment using full-strength white vinegar. Both lemon juice and vinegar are mild acids.

Hydrochloric acid, a powerful acid used in some toilet-bowl cleaners, is also sometimes used on rust stains, but I consider it too dangerous for do-it-yourselfers and do not recommend using it.

Q. My concrete driveway has a lot of pock marks and flaked areas, which I think were caused by using rock salt as a de-icer. The driveway looks pretty bad. What can I do to salvage it?

A. The first step is to pressure wash the driveway to remove any traces of the rock salt.

It is seldom possible to repair a badly deteriorated concrete surface by simple patching, but if there is only minor surface damage, it might be possible to improve the appearance with a concrete resurfacer, which is spread over the surface in a thin coat.

You should consult with an experienced concrete contractor to determine if it is possible to resurface. If that won’t work, the other option is an overlay, which means adding another layer of concrete to the existing driveway.

If you decide to use an overlay, it is extremely important to pick an experienced contractor. The surface of the existing concrete must be carefully prepared; it is generally roughened by one of several techniques, such as sandblasting or machine milling.

Good surface preparation is important to provide a good bond between the old and new layers of concrete; if there is a poor bond, the new layer can crack and deteriorate.

An experienced contractor is also needed to determine the thickness of a proposed overlay. Some overlays are several inches thick and might require reinforcing with steel mesh or rods.

Finally, the new layer of concrete must be allowed to cure properly before it is used.

Future de-icing should be done with materials that are easier on concrete, such as calcium chloride.

Q. I want to repaint the wood around my garage door. The wood has some knots in it that appear to be loose. Should I caulk around the knots with a silicone caulk before repainting?

A. You don’t want to use silicone caulk, because most silicones cannot be painted.

You could use an acrylic caulk, which is paintable, but if there are gaps or cracks around the knots the best bet is to fill them with a vinyl wood-filler suitable for outdoor use. Let the wood filler dry, then sand it smooth. Knots will often “bleed through” paint unless they are primed.

I prefer a shellac-based primer, B-I-N, which is sold at many home centers and paint stores. When the primer is dry, you can paint.