|MadSci Network: Physics|
I was not the person who answered the previous question. I am not aware of the effect you mention. The signal in a magnetic card read head is quite sensitive to the spacing between the card and the head. Hence putting a thin spacer on the card will reduce the signal and so also reduce the signal to noise ratio. Depending on a lot of details, this will tend to increase the errors in reading the card, not reduce them. The card can be cleaned with a damp cloth and should work if it is not damaged somehow. The standard stripe can be rather easily demagnetized. A common rubber refrigerator magnet will do it. I need to know more details of what you observe before I can comment further.
Try the links in the MadSci Library for more information on Physics.