Hi Temprite
I got first hand experience of the rust and the corrosion that it causes, I use a product called TECTYL, mostly used in the automotive industry to treat the underside of vehicles as a sealant, goes on and sticks like bubblegum to a monkeys backside, last about 2 years in the sun if sprayed nice and thick, speak to your panelbeater about what they use, its oil based just runnier than tar, but same principal