weathering steel planter boxes | Planterscape Blog

3 Steps to Weathering CorTen Steel Planters

By Metal Planter Boxes, Uncategorized

All this rain has got me thinking about the process of weathering CorTen steel planters. Rain is part of the acceleration! If you love every bit of the coppery bronze patina of weathered CorTen steel planters, but could do without the time of nature to transform your CorTen steel, you might want to experiment a bit.

Let me share my process that I have used in the past that has delivered great results!

Step 1:  Making the rusting concentrate.

You will need the following to begin making the rusting concentrate:

1 qt. glass canning jar with metal ring (you won’t be using the lid)
disposable measuring cup with ounce markings
plastic wrap or sandwich bag
24″ of copper wire
2 oz. Muriatic Acid
face mask
chemical protective gloves
thick clothing

Before beginning, it is important that you are in a well-ventilated room.  You will be working with Muriatic Acid, which emits strong fumes that can damage the lungs and can burn your skin without the proper protection.  Read and follow all safety instructions on the Muriatic Acid bottle, including but not limited to wearing chemical protective gloves, proper face mask, and thick clothing.  Muriatic Acid is highly caustic and will corrode anything, so be sure your working area is free form anything you do not want destroyed.

If you are still up for this journey, and made all safety preparations, begin coiling your wire very tight. Coiling it around a pen or marker will make the coil tight and small.  Set aside the pen or maker, if you used one, and add the coiled wire in the quart glass jar.  Carefully measure 2 oz of Muriatic Acid and pour into the quart jar with the copper wire.  Cover the quart jar tightly with plastic wrap or slide a sandwich bag over the top.  Take the metal ring (without the lid) and screw it on the quart glass jar over the plastic.  Poke many holes in the plastic for ventilation.  Without ventilation, the concentrate may explode from the gasses, so it is important to poke holes in the plastic.  Appropriately label the quart jar, indicating it contains toxic substances and keep it in a safe place and out of reach.  You can purchase safety labels at www.mysafetylabel.com to ensure your container is properly labeled.

The concentrate should sit for 1 week.  When you return to inspect, the concentrate should look like stout beer and the copper wire should be partially if not fully dissolved.

Step 2:  Making the rusting slurry.

For this step, will need:

20 oz. Water
1 oz. of the rusting concentrate
Disposable measuring cup with ounce markings
Small thick plastic container, like a container used for painting
A plastic spray bottle with no metal parts, that holds at least 24 oz.
face mask
chemical protective gloves
thick clothing

Again, with proper safety preparations completed, mix 1 oz. of the rusting concentrate with 20 oz. of water in the plastic container.  This will make a 1:20 oz. ratio of rusting slurry.  Carefully pour this slurry into the plastic spray bottle.  Your rusting slurring is now ready for application.

Step 3:  Applying the rusting slurry.

As before, continue with all safety preparations and proceed to do this step outside.  Also, take care in selecting an appropriate area to spray your planter, as you are still working with Muriatic Acid, and while diluted may drip onto the surface beneath.  A gravel or soil surface can work fine.  Set your sprayer to a fine mist, and begin to spray the surface of your planter.  Avoid heavily coating the planter causing excessive dripping.  Spray the planter thoroughly but apply only 1 coat at this stage.  The 1:20 ratio is a strong ratio and in no time, your planter will begin rusting.  I would not recommend a second coat at this dilution ratio.  If you want to have a slower go at it, to more slowly control the rusting, you can play with the ratio of concentrate to water, increasing the dilution anywhere up to 1/100.

The plastic bottle will begin to erode eventually given the corrosive properties of the rusting slurring.  Take all proper precautions to dispose of all the containers and liquids if not fully used.  If keeping any remaining corrosive concentrate or slurry, be sure to store safely (in appropriate non-corrosive containers) and label properly.

Here are photos demonstrating our success with this rusting slurry:

Day 1:  Application of rusting slurry


Day 6:  Planters achieving a very nice patina after only 6 days….


Day 45: Wonderful bronze hues achieved on the very same Bowery rectangle planter.   The patina process will continue to deepen the warm copper earthy tones.


Learn more about CorTen steel on Planterscape’s FAQ page at https://www.planterscape.com/pages/faq and enjoy this rainy summer for weathering your steel planters!