I live in Ohio and I use a product called wool wax it doesn’t wash off like fluid film and it creeps up the metal I applied it once and only do touch ups I run thru carwashs with undercarriage sprayers also
Thus maybe a little simple but I would go to the parts store and get a glow plug controller relay for a 87 6.9 ford diesel and install it to the glow plugs that way it’s automatic with the switch and your not over heating them