I’ve been using the eBay Air Polisher (shown below) for a few years now and overall it has performed relatively well, except for one major problem. The problem is that one of the o-rings wears out very quickly and causes water leakage. To fix the problem you will only need to replace the o-ring with a high quality version and then lube the o-ring area with a very small amount of lubricant after every use.
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Autoclavable Silicone Lubricant ($18.95)
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Photos
This is the air polisher fully assembled.
Autoclavable Silicone lubricant
Tip detached from the main body of the air polisher. Arrow pointing to the o-ring that needs to be replaced
Mark Frias, RDH