I keep my Evapo-rust covered tightly at all times, even when I am using it. Sometimes I think it matters how much water is in it compared to the active ingredients, just a gut feeling, but based on some results. I also think that Evapo-rust should have just enough detergent in it to make a few floating bubbles if you stir it briskly. This I know makes a difference. If you put oily or greasy parts in the Evapo-rust, it uses up the detergents that came in the Evapo-rust. Then the working ingredients cannot get to the rust easily, too much surface tension. If it seems flat, add a few drops of liquid dish washing detergent and see if you get bubbles. You do not want it sudsy or frothy, just enough to make some bubbles. It is a really good idea to remove all grease and oil, and loose rust, before putting the parts in Evapo-rust, if you want continued good results. Eventually, the chelating agent in the solution is used up by dissolving rust, and then it is dead. Until then you can stretch the life of it significantly, in my experience, if you take care of it and use it wisely.