As for rejuvenating: As Al said -- if you could simply add water, then when you put it the water....

A few years ago we tried Micron's ablative paint on our depth sounder (B&G Hydra) by "accident" when I had the bottom sprayed by someone. The same thing happened as when we did it before with other bottom paints: I would get some false echos. I carefully removed the bottom paint after I pulled the transducer (B&G has that option for both the depth and speedo units). After we switch to a VC offshore (a harder, non-ablative paint because we didn't want to create a cloud of "stuff" when we had the bottom wiped by a diver), it seemed to work okay on the depth sounder.

Assuming you have a sensor that that can be pulled up into the hull and "plugged", I wouldn't use paint on a speedo's paddle wheel sensor. I just pull them and use my fingers, a paper towel, or (occasionally) an old soft toothbrush to gently clean the sensor. Whenever were going to be port for more than a few days, I pull the sensor and plug-it. (The B&G has a neat flap system which limits the salty surge of water.) Do I recall, you're is in forward shower pan?