Probably 10 or 15 years ago I put in a new Sherwood. I notice is seems very rusty except on the cover and so their is a little rust on the inside edge near where the impeller spins. I have a good o-ring in in place and I change out the impeller yearly. I was thinking of replacing the old water pump. What do folks think? Should I? Is that something that simply must be done every once in a while? Is it a professional job or can I do it? Do people prefer the Sherwood or the original Oberdorfer whose impeller cover is much easier to take off since the screws are not in back of the cover like the poorly designed Sherwood?