Galvanised metal consists of steel or iron that is coated with a thin layer of zinc, which protects the underlying metal from rusting through oxidation. If you have an item made from galvanised metal and you want it to oxidise, you must first remove the zinc coating. This is often done to give galvanised metal an "aged" or "weathered" look. The most common method for removing the zinc from galvanised metal is to use an acid solution to dissolve the zinc.
Decide on the type of acid you will use to remove the zinc. One option is to use vinegar, which contains acetic acid. But vinegar contains only about 10 per cent acetic acid, so using it to remove zinc from galvanised metal will take longer. If you want to get the job done more quickly, you can use muriatic acid, which is available at hardware and building supply stores. Muriatic acid is actually a solution that contains about 25 per cent hydrochloric acid. It will dissolve the zinc more quickly, but you must avoid breathing it and getting it on your skin.
Prepare your work area. If you are using muriatic acid, use it only in well-ventilated areas, preferably outdoors. Make sure you wear gloves when handling muriatic acid and keep water and some baking soda or ammonia nearby in case you need to quickly neutralise spilt acid.
Put the acid solution in a container with a lid if your are removing the zinc from a smaller item. Soak the item overnight if your are using vinegar. Soak it for about an hour if your are using muriatic acid.
Apply the acid to the galvanised metal with a brush or sprayer if you are removing the zinc from larger items such as sheet metal. If you are working with muriatic acid, buy a cheap brush or sprayer that you can throw away afterwards.
Wait overnight for the vinegar do its work, and wait about an hour for the muriatic acid to do its work.
Clean the acid from the metal. If you are using vinegar, clean the metal with water. If you are using muriatic acid, use baking soda or ammonia to neutralise the acid then use water to clean the metal.
To get uniform oxidation of your steel or iron, you must remove all or most of the zinc. You may need to repeat the steps if all of the zinc does not come off the first time.