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.

Use something else for DIY. Cut up your cards, kaing sure you cut the
magnetic strip, the chips and the number. Then place half of the pieces in
two separate bins.

Christian.