If at all possible, submerge the entire part in the EvapoRust. If some of the piece is sticking out of the solution, an etching line can be created. If you must have a section of the part sticking out of the solution, you can put paper towels/shop towels around the part at the water line. This will help stop any etching line from forming.

Also, I soaked one of my window mechanisms in it. The spring on the mechanism had a lot of surface rust on it. After removing the mechanism from the EvapoRust, the spring was cracked. Those are the only two issues I've had with it. If you leave it submerged too long, it will remove factory paint markings when you rinse it off.

About a month ago, I cleaned up an 8 3/4 center section with it. I wish someone made a heavy duty plastic "form" that the center section would fit into. Then you would only need a small amount of EvapoRust to completely cover the part. You need a lot of product to cover a center section if you are using a bucket.