Ask the Builder: Your rust removal questions and my answers

Oct 24, 2023

Sat., Nov. 5, 2022

Every week, readers reach out to me with fantastic questions about a column of mine they have just read.

Often they ask why I didn't delve deeper into the topic. The answer is that newspapers – the medium for which my column is produced – have space limits that I need to adhere to.

About a month ago I wrote about removing rust from my own toilet. I got an avalanche of questions – so many that I felt a follow-up column was in order. Let's get started.

The most frequent question was, Will oxalic acid harm the plumbing pipes in my home? A frequent related question: How does one neutralize the acid?

Weak oxalic acid will not harm PVC or ABS piping. Plastic drainpipes are generally immune from any and all weak acids. Cast iron drainpipes and acids don't play well together, so it's important to neutralize the acid before you allow it to flow through the pipes on its way to the city sewer or septic tank.

You neutralize an oxalic acid solution adding ordinary baking soda. Start to pour some into the solution. It should bubble. Stir carefully and continue to add more soda powder until the bubbling stops. The best way to confirm that the solution is neutralized is with litmus paper. Make sure the solution has a pH of 7.0. Adding lots of water to the solution will also dilute the acid.

Some readers asked if the oxalic acid would hurt a septic tank and the biologic activity inside it. Well, we know for a fact it won't help it! The safest thing to do when cleaning anything using an oxalic acid solution would be to prevent the solution from going down the drain. Capture the acid solution and take it outdoors to pour onto the ground once you’ve neutralized it.

Others asked how safe it is to clean acrylic tubs and showers, chrome faucet parts, ceramic tile and all sorts of surfaces in a normal bathroom that might have rust stains. In my experience, an oxalic acid solution doesn't harm anything other than polished marble. Keep any and all acids away from marble.

Before you apply an oxalic acid solution to any surface, I recommend that you mix up a small amount of the solution and use a cotton swab to apply a drop of it to the object. Let the acid sit for about an hour. Before you apply the drop, make sure that the area is clean and dry. Choose an out-of-the-way spot in case the acid does damage the sheen.

What about using oxalic acid outdoors to clean rust from vinyl siding, brick, mortar, concrete and so forth? You’ll get rapid results when cleaning vinyl siding, but it takes quite a bit of time to get great results on rock, concrete, brick and mortar. This is because the rust can soak into these surfaces. You just have to be very patient and try to work in the shade so the oxalic acid solution doesn't evaporate.

Oxalic acid solutions are the go-to cleaner for exterior redwood. If you recall your high school chemistry, you know that acidic solutions have a pH lower than 7. Caustic chemicals like many oxidizers have a pH higher than 7. Redwood can turn black or darken if you use anything with a pH higher than 7.

This is why you never want to use chlorine bleach or any other oxidizer to clean redwood. All that said, always test using an oxalic acid solution solution to clean your redwood or any other surface. I can't stress that enough. You can't hope everything is going to work out fine. It only takes an hour at most to run a test.

Where can you buy oxalic acid? Fortunately, it's cheap. I bought mine online from Amazon. I got a sizable pouch of the white powder for less than $15. You may find these pouches at a local or chain hardware store.

Always read the instructions about how to mix, use and store the unused acid. My pouch is sitting on the bottom shelf of my refrigerator right now next to cartons of milk. Oxalic acid powder likes to be stored cold if at all possible. If you’ve got kids in the house, be very careful where you store it. You never want the possibility of a child mistaking the acid for powdered sugar. It looks just like it.

What can I help you with? What issues around your home worry you? What do you want me to discuss in my upcoming columns? Go here and tell me. Be sure to type the word GO in the URL: https://GO.askthebuilder.com/helpmetim

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