Did try to adjust in open field and have no rust issues. Im thinking defect in antenna. Did not come with scratch off card and looks like package was restapled. Bought it from H&Y.
The PL5000 has a wide band performance rating. Are you using it for just 11 meters? If used for just 11 meters it should be showing a nearly flat swr of hopefully 1:1.
On my mini SUV I had to trim almost 7 cm off my whip for the freqs I wanted to use. Check the swr on channels 1&40. I have the advantage of having a multi band radio, meaning I see a much more drastic change of swr from 25.515-30.315 MHz.
Maybe the big dogs might can pitch in and help you to solve this issue.