Thanks for everyone sharing their knowledge and experience in this thread. As a consequence, I have modified my ultrasonic cleaning process in several ways:
1. I extended the de-gassing cycle to the time it takes to warm up the water bath to 35 C, and I now de-gas before each use, after the device has been sitting unused for a day;
2. I added a capful of surfactant in the 6L tub (I’m using Photo Flo 200);
3. I extended the cleaning time in the warm ultrasonic bath, to approximately the time it took to de-gas and warm up the water bath;
4. I slowed the rotation of the motor to about 3 cycles per minute (this is using a low voltage adjustable DC adapter set at its lowest 3V), which is as slow as my adapter will turn the motor;
5. I added a "rinse" process: I just use a sprayer filled with distilled water to spray both sides of the LP after the ultrasonic bath.
I still use 1 oz of 91% isopropyl alcohol in the 6L of distilled water. . . and this is for the VEVOR 40kHz ultrasonic machine.
I’ll find out how much of a difference this makes from the way I used to clean, which was just throwing the LP in the warm ultrasonic bath for a few minutes at a high rotational speed, and call it a day.
I will keep an open mind, and consider that it is possible that I'm just being obsessive over an issue that won't make much of a difference at all. . .but perhaps it may. . .my system is sensitive and detailed enough where I should be able to detect an audible difference, if there is one. . . although for most users on this forum, just a simple wash in the ultrasonic bath would probably be sufficient.
Thanks!