Inspection of the left radiator for foreign matter clogging the radiator outlet is a part of the ECM replacement described in Honda Service Bulletin #13. It states, "If there is any foreign matter clogging the radiator outlet, you MUST replace the left side radiator. Inspect the wiring harness in the left side radiator area for damage. Look for any chafing or wires that are being pinched. Wrap any affected areas with electrical tape".
Apparently, the wiring harness on some GL1800s was chafing on the radiator and causing an ElectroChemical reaction which is the cause of the solids building up in the radiator. I personally inspected my radiator during the ECM replacement and there was no build up but the wiring harness was beginning to chafe.
leland watson I remember reading somewhere about crystal forming within the radiator due to a small current originating from a wiring harness which was next to the radiator. Probably not much help. Good Luck.