Have you tried putting citric acid inside the fount? I don't know where I went wrong with my 335 lantern but I found out the hard way there was a type of rust protector on the inside. I didn't have a tool to remove the check valve so I worked at it for a 1-1/2 days of spraying carb cleaner into the valve to clean it up. After I got the valve working I put a citric acid solution on the inside of the fount to clean up any varnish because I was rather certain the previous owner left it full of pump gas until it evaporated dry. When I was rinsing out the tank all this paint came off the inside of the tank. Internet said using both carb cleaner and citric acid was okay. I hope it doesn't rust away too quickly now.
