Molasses is good for cleaning rust from steel.

Caustic soda is good for stripping paint from plastic parts, it is available from Bunnings and dilute as per instructions. Caustic soda won't hurt the plastic no matter how long you leave it in.... I've left a fascia in it for three months to get all the paint layers off it that previous owners had applied.

caustic soda burns like hell if you get it on your skin so wear protective gear and keep your tub covered and away from children or pets. I was using a wheelie bin in Melbourne but will have to find something better now we're in Parkes..