The 'cleaning' processes Mr. Crossley mentioned seem rather time consuming and possible difficult to accomplish at home. And you'd need yet another RH indicator.
I'm not sure you should expect to see
very high RH values
just because it is raining. Nor do I think a high
rate of rain always results in a high RH. In fact, you would expect to see nearly 100% RH even without precip if you had heavy fog at the sensor.
While high RH is related to precip, the actual location of that very high RH is where the precip is forming; in the cloud. Heavy precip is often seen in thunderstorms and the temp on the ground can be considerably higher than even the base of the Cumulonimbus cloud. If the RH of the air below the cloud is low enough and the base of the cloud is quite high, it is possible that the rain will never even reach the ground (VIRGA).
While your sensor may indeed be
inaccurate, I think you would need an identical or
tested, calibrated sensor to compare it with. And, as far as I know (that's a huge variable!), RH can be a fairly localized measurement. I would think being close to a large body of water, especially with a shoreward wind, would have a higher RH than the opposite wind. OTOH, the RH in the middle of a large forrest will also have a higher RH than the same area covered by grasslands and certainly urban areas.
I
do hope you find a way to verify the RH values in the least costly manner!