Hi!
I seldom use locking agents but I really should!
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I use them on places where I do not want the part to get loose such as the shutter speed adjustment gears of the F/F2 and the RF cameras
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I used locking agents on all of the external screws of Buddy's lens because I know how hard he uses his gear
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I use a red lacquer thread locking thing called "ねじの救急車" or "screw ambulance". nail polish is OK,too. It is used by the repair shops back home to save money.
I rarely use nail polish.