It's safe to say the weather radars in Metros and similar vintage radars shouldn't be relied on if you can't see outside or want to see beyond the crap in front of you.
As Judd said, water leaks and paint repairs are typically the culprit. These things only get worse with age.
A C&T captain taught me to always do my homework on where we were going and what might happen, with storms in particular, from multiple sources. The online weather radars these days are gold - one wonders why Metro guys can't access them from the cockpit.