Hmmm, I do hope this does help. It's the middle two on the rear that don't work. Any suggestions on the best way to 'scrape' off the paint and what to use so as not to damage them further?
Sorry for responding so late.
Apparently you can scrape the paint off with a screwdriver. Since the sensors have an aluminium surface, they can handle some abuse. You will need to scrape off the corrosion on the aluminium anyway, so a screwdriver seems to be the right tool for the job.
Just make sure you only scrape the paint off within the rubber ring. When you repaint the sensor, make sure no paint goes onto the ring or you can throw the sensor away.