I like to keep my old bank cards intact and use them for jobs around
the house. My worry is that the data on the magnetic strip can still
be read.

What is the best way to destroy that data without messing up the card
for D-I-Y work.

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Does a domestic microwave detroy data on a magnetic strip?

I tried using some of those strong magnets taken from a hard drive.
I put one above and one below the bank card and then slowly drew the
magnets along the magnetic strip. Is this enough? Is it better to
do this relatively quickly or relatively slowly?

ISTR something about vertical and horizontal magnetic polarization
and maybe this is something I have to deal with in case it makes the
magnetic data harder to destroy???

Is it better to scratch the magnetic strip with a sharp point?