Carl-Johan
Contributor
Some Canon OEM housings have a mould mark that has to be sanded. This is fully explained in this thread.
Besides, Canon OEM housings have been accused of sticky buttons. With these indications, you can avoid sticky buttons.
Both my Canon housings have mold marks and neither have flooded. Canon must have made millions of housings over a long period of time, if the mold marks is such a big problem they would have fixed them totally by now.
Canon, nor any other maker, obviously do not watertest every housing they make so it's best to do it yourself before putting the camera inside. The shallower depth the better, i used a sallad bowl and left it there overnight.
Matts tutorial is very good but don't forget to also put grease on the actual plunger, not only the oring needs lube. When putting the oring back use the back of plunger, the button so to speak. It fits good, is rounded and will not harm the oring.
The maintenance is done every year, after about 70 dives. That might be overkill but i actually enjoy the work.
Remember, "if it aint broke, do not attempt to fix it"