Check for electrolysis. Was talking to a radiator manufacturer today at the Roadster show. When he sees "gorilla snot" he calls it in the radiator it's usually caused by electrolysis.
check electrolysis by simply hooking up a multimeter to the battery ground and dangle the positive side in the fluid in the radiator (not touching anything). Do it with the battery hooked up and car running and then not running,
Should be under .3 volts (300 mV).
Mine pictured below is .076 volts ( 76 mV) with distilled water, 1 bottle of Justice Brothers Super Radiator Cooler and 1 bottle of Justice Brothers Cooling System Protector.
