The bung at the bottom would prevent the glycol from going bye bye.
I would add the glycol AFTER plugging the thru hull from underneath. Since it appears that glycol is slightly heavier than water, it will settle at the bottom inside the plugged valve, displacing the sea water.
I would add the glycol AFTER plugging the thru hull from underneath. Since it appears that glycol is slightly heavier than water, it will settle at the bottom inside the plugged valve, displacing the sea water.