Most use a 3:1 up to 5:1 ratio ( water to FC stock solution). For more delicate etching and hamon work, it is usually 10:1 up to 15:1.
When mixing the stock solution from dry powder, use caution.
Mix it outside and stay upwind. It gives off a chlorine gas when you first mix it. It also can get very hot while mixing. Add the powder slowly to the water. Don't mix it in a thin plastic jug ( milk jug)...it might melt. Mix it in a heavy plastic bucket, like a drywall bucket. When cooled down, pour into a plastic storage container and wash out the mix pot.
FC should be mixed to make a stock solution. You add one pound to one quart or four pounds to a gallon of water. Distilled water is best, but any water that is not full of minerals (I use bottled water) is fine. Let it sit for a couple days before using it. Dilute the working solution from this stock . Three parts water to one part stock is what most folks use.
If you are just making up one gallon, skip the stock solution, and add one pound FC powder to a gallon of water. If making up a larger batch, one gallon of stock solution is easier to store than four gallons of working solution.
Again - Mix outdoors and don't breath the fumes when it is dissolving. Wearing gloves and a face shield, is a good practice.
4# of powdered FC sells for about $30 on eBay. It will mix a gallon of stock which will make four gallons of etching solution. Immensely cheaper than Radio Shack or other pre-mixed suppliers.
