Oxalic acid advice

First time attempting rust removal from AC unit on white vinyl siding. I ordered oxalic acid powder. Curious as to mixing ratios, also can I put it into the same pump up sprayer that had sh in it prior as long as it is rinsed out? Also can I mix the batch the night before, any help I would appreciate!

I usually mix Ox on the job site just before I need it. 8 oz to a gallon. Warm water preferred but I’ve been just fine using cold.

I use f9 Barc for most of my rust removals. I charge a premium and barely use any product as I mix it 1 gal to half a gal water.


Okay awesome so 8 oz Of ox to one Gal of water…

Oh, I have something to say about oxalic…

Couple years ago when I started, a customer asked if I could get some rust off their concrete patio. Per instructions I mixed 8oz to a gallon of warm water. Suddenly my eyes and lungs were burning like fire and I had to back off from the bucket. One whiff of the stuff was like tear gas.

I asked why that would happen, it was pointed out I may have had some window cleaning residue in the bucket (Dawn and water) but I assure you it was well rinsed.

I won’t use the stuff again. It says right on the tub that it gives off poisonous fumes and to wear a respirator.

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Wow that’s pretty sketchy. I did purchase a respirator when I ordered the ox but still.

Also, can I apply ox, rinse, than apply house mix ?

Doesn’t matter if there’s other stuff (well it does acid and alkaline is very dangerous) but oxalic itself is nasty stuff! Treat it like, well, like its acid!!!

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Do you mean one ounce per half gallon? That works for you tho? I use it 50/50 and put it in a small spray bottle. Most rust stains pretty small and it will still last forever even 1:1

I think I did this right first time quoting something but she said that it’s alright to batch mix oxalic. It wont lose its potency like sodium per carbonate or sh will it just falls out of solution and needs to be stirred again


Anything oxalic or acid related should have its own sprayers and holding containers. Treat it as a whole different system to your other chemicals.


I am confused by this thread. I think of oxalic acid as the the thing that is called an acid but is barely hazardous. It’s not fuming or potent like HCl. It also sits on retail shelves for months as wood brightener and isn’t known for losing its potency because it doesn’t gas off like SH or per carb.

The only annoying thing about oxalic is that it doesn’t go into water very easily or at a high rate. I typically add in some phosphoric acid to make it a little happier in the water. And phosphoric also assist with rust removal.

The oxalic is a little dusty, it can form a cloud when you dump it, but that’s the only reason in my opinion to use a respirator with it. My chemical mixers used this often and I was only concerned about the cloud, not at all while it was wet.


Thank you for dispelling the myths.

A product used by many people on the forum, and regularly recommended, contains a stronger acid in it than oxalic, and has another nasty ingredient (F9 BARC). The rubber scrubbers use stronger acids than oxalic to clean $$$$ trucks. People on here recommend muriatic acid and that is stronger. I do believe that PPE should be used with every chemical, it just makes sense in the long term. If using a lot of oxalic or mixing up a large batch, wear PPE and have a 5 gal bucket of clean water nearby. Just splash it off your skin if you get “tingly”. I do this an I er fin naw prblms…


Sorry, y’all. I was wrong! :man_facepalming:
I deleted my comment above.
I must’ve been thinking of sodium percarbonate.

Is it possible that @dcbrock is allergic to oxalic? I never use a respirator or anything and I’ve never been bothered by oxalic even if I’m spraying it up high and it mists down on me. You can get it on your hands and it barely stings. It looks to be considered a weak acid on the acid strength chart.

You can use citric acid for wood brightening @dcbrock. I did a test and it seems to work just as good. Although, the fence I tested it on wasn’t that old. It might not do as good of a job on an old dark fence. I doubt it will bother you as bad. Not sure how it would work on rust removal but probably worthless there.

Use F9 and charge accordingly.

Took about 30 minutes from roll up to roll out.


Were you huffing it or something? :wink: That’s not a normal reaction!

What do you use after washing a deck to bring the ph back down? Or are you skimping? :face_with_hand_over_mouth:

It doesn’t bother me unless I’m spraying up and get it on my face. Then it stings a bit. I’m sure it’s not good to get it on the skin. But neither is SH!

Everything worked out fine. Mixed 4 oz of ox to half gal of Luke warm water. No cloud of dust. Wore proper ppe and pump sprayer.


I am not trying to step on toes around here, but I know what I experienced and it was really toxic when I got a whiff. Here is what the warning label says, nasty stuff.

I can try mixing some up again in a virgin container but I seriously doubt my bucket had any residue that would create mustard gas like that.:flushed:

I guess it’s possible to be allergic. This label is actually proving my point that oxalic isn’t bad. It’s a little known fact that I went from mixing chemicals in a lab to being the regulatory person for a well known chemical company. One of my main job duties now is writing labels and SDS’s. I can tell you with complete certainty that those phrases are coming from the SDS hazard codes, and they aren’t the bad ones. This says MAY cause burns, the bad ones are H314 “causes severe skin burns and eye damage”…H318 is also not fun with “causes serious eye damage”…this label isn’t claiming to actually cause burns and or irritation. There is a huge difference when the word MAY is brought into the regs, so in SDS land it’s a mild warning.

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@dcbrock could have been the jalapeño peppers you ate earlier.

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