Putting your DIN regulator into a rental DIN valve is in general a bad idea.
The weakness of the DIN connection is that it relies on matting threaded connection with other threads. The threads are exposed to the corrosive salt environment and in heavily used environment they can receive a lot of wear.
There is no easy way to inspect the back side of DIN female threads to see their condition. That is the loaded side and is the side that receives all the wear. Chrome missing from DIN valves threads is very common for heavily used valves, especially in warm Caribbean ocean use.
I started using DIN connection in the Caribbean almost 40 years ago (long before it was even called DIN). I used to think they were great until I started having trouble with the wear on the threads, salt corrosive environment with some from heavy use. Once the chrome is gone, inside the threads, the connection tends to bind and it is hard to remove the regulator after a dive (after a pressure cycle).
My recommendation is to save your DIN regulators to be used with your nicely maintained personal tanks. The only exception would be if you are renting tanks that are not used often, like technical doubles tanks or other lightly used tanks. That would normally only be offered by a specialty technical shop, that hopefully maintains its gear more than the average.
Screwing your nicely maintained personal regulator into a heavily used tank valve is a good way of putting excessive wear on the threads of your regulator.
If you need to use a DIN valve, I would recommend as a minimum to clean the female threads with a nylon brush (a tooth brush or bottle brush) every time you are going to use it. Very small particles in the threads (sand or even salt crystals) can abrade the chrome surface on your regulator.
A large particle, like a grain of sand, can actually cause the threads to lock when you pressurize the connection. After you depressurize the connection, you may not be able to disconnect the regulator as easily as you thought.
Also, carefully inspect that the valve opening is perfectly round and there are no marks of external impact. If the regulator doesn’t screw very easily, with no noticeable friction, do not use it. It may not seem that bad at first, but the worst damage will happen after you pressurize it.
Personally, I have gone back to using mostly yoke connections and for a rental Caribbean tank, IMHO the yoke is a far better choice. A yoke connection is a lot more forgiving in a corrosive environment, especially in the long run. And if properly used, my experience has showed me that it is just as secured.
Note: I have posted similar observations/ recommendations in the tank/ valve section, but it has been a while. I hope this information is helpful.
