Before you change the rack, usually the splines get soft aluminium debris from the wiper arm stuck in the grooves! carefully use a stanley knife blade to chase out all the debris from each groove, then fit a new wiper arm... you
can heat the new wiper arm prior to re fixing to the spindle, to ensure a tighter fit, but getting the nut on tight before it cools may be a little tricky, so have all you kit at hand

HTH
Thom