Well my heater box had been home to a nest of mice, judging from the tidy rectangle of fibers and fuzz that was in the bottom of it, with the little cove where the babies would have laid. Maybe that contributed to the heavy rust that was on the flaps and inside the lower box area. And the drain pipe was plugged, so that's probably more water coming in. The outside of the box had lots of rust too, once you started poking at it. So I scrubbed at it with a wire brush and sandpaper and coarse steel wool. After an hour and a half it was better, but I still had a long way to go and some of the corners were just not accessible to the tools I was using. Googling "rust removal" I found somebody telling about putting iron bars in a bucket of water with wires and a battery charger. I tried it (there's more to it, you have to find a site and read up) and it worked. The rust came off and so did the paint. So now one half of the heater box has a coat of POR-15 and a coat of primer on it, and the other half is in the plastic tub with little bubbles rising off of it...and tomorrow I should be able to clean and paint that half too. Pretty neat. Labor saving and very effective.