I just use white vinegar, full strength, I heat it until warm and let it set, then I wipe it, if not loose yet I warm it again. In the cold weather I warm it until it starts "steaming off". While it is not really steam it looks like steam coming from the pan, in reality it is likely 100-120F. I do not drain the pan until the pans will wipe clean. I use a weed burner torch under the pan to heat it, keep moving the flame to warm it all. Once it will wipe clean, I drain the pan, wipe it down and then flush using permeate (if you don't have permeate, use good soft water), I have a 3 gpm pump that feeds my tankless water heater and I flush with a hose and make sure it is totally clean.
A white film shows that you most likely have mineral deposits on the surface.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.