Any volume of water has a limited capacity to suspend salts in a soluble state so too much salt, in this case, OA salts, and it will precipitate out of its aqueous state and deposit onto the surface of your parts as a scale. Higher concentrations of OA just means greater capacity to complex with the available ions it has affinity for. In our case that is iron. This being said, as was stated above, less salt to start with, perhaps 1 tablespoon per gallon, should be adequate in providing enough material to complex with available iron oxide without leaving behind a scale. Previous posts are also correct by suggesting the use of warm water in that it will effectively speed up the ion exchange AND help with maintaining solubility during the process. The ideal set up would be a continuously heated and agitated solution but that takes dedicated equipment and space. Hope this helps and good luck.