Guide . updated 2026-11-14
How much oxalic acid per gallon of water
Short answer: for spraying rust stains, most people do well with about 5 to 10 ounces by weight of oxalic acid dihydrate per gallon of warm water. That is roughly a 4 to 8 percent solution. Start at the lower end on painted or delicate surfaces, go to the higher end for concrete and heavy irrigation stains, and always test a small spot first.
Strength by surface
| Surface | Starting mix per gallon | Dwell time | Notes |
|---|---|---|---|
| Vinyl fence and siding | About 5 to 6 oz | 5 to 10 minutes | Keep it wet, rinse well, rinse plants below |
| Concrete, pavers, driveways | About 8 to 10 oz | 10 to 15 minutes | Repeat rather than going stronger |
| Fiberglass and gelcoat | About 5 to 6 oz | 5 to 10 minutes | Rinse thoroughly, avoid letting it dry |
| Wood (graying and iron stains) | About 5 to 8 oz | 10 to 20 minutes | Rinse and let dry before sealing |
| Chrome and painted metal | About 4 to 5 oz | 2 to 5 minutes | Short dwell, rinse, dry |
These are starting points, not guarantees. Surfaces and stains vary. Never use oxalic acid on marble, limestone, travertine or other natural stone, which acid will etch.
Why the advice online disagrees
If you search this question you will find answers from a quarter cup to a full pound per gallon. Both can be right for their job. A federal historic preservation procedure for poulticing iron stains out of masonry uses about a pound per gallon, because that paste sits on the stone for hours. A quick spray on a vinyl fence needs a fraction of that.
The other problem is measuring by volume. A cup of fine powder and a cup of coarse crystals weigh different amounts, so two people following the same recipe can mix very different strengths. Weigh it if you can.
Cold water is the hidden problem
Oxalic acid dissolves much less in cold water. Fill a backpack sprayer from a cold hose or well and a strong mix can leave undissolved crystals in the tank, which clog filters and leave you spraying a weaker solution than you think. Use warm water, mix until it is clear, then top off.
Safety when you mix it yourself
Oxalic acid is harmful if swallowed and can cause serious eye damage. The dust irritates your nose and lungs. Wear a P100 or N95 respirator, sealed goggles and gloves while measuring powder. Never mix it with bleach or other cleaners. Keep children and pets away and rinse plants and grass with plenty of water after spraying.
The pre-measured option
SpraySticks rust remover sticks and pods take the measuring out of it. One stick in a 16 oz bottle makes a light mix and two sticks make a full-strength mix. One gallon pod makes about a 7 percent solution in a gallon of water, and four pods fill a standard backpack sprayer. You still need gloves and eye protection, but you never handle loose powder or guess at the dose.
Common questions
How much oxalic acid do I mix per gallon for concrete rust stains?
A common starting point for spraying concrete is about 8 to 10 ounces by weight of oxalic acid dihydrate per gallon of warm water, roughly a 6 to 8 percent solution. Heavy irrigation stains may need a second application rather than a stronger mix.
Why does advice range from a quarter cup to a pound per gallon?
Different sources describe different jobs. A pound per gallon is used for thick poultices that sit on historic masonry, while light household sprays are much weaker. Measuring by cups also varies with crystal size, so weight is more reliable.
Does oxalic acid dissolve in cold water?
Slowly. Its solubility drops sharply in cold water, so strong mixes can leave crystals at the bottom of a sprayer filled from a cold hose or well. Use warm water and stir or shake until clear.