Deck stain is thin, so it splashes and spatters easily, making it challenging to do a really neat job. Unfortunately, it's all too easy to get deck stain on your aluminum siding while staining your deck. Your success in removing deck stain from aluminum siding will depend on how fresh the stain is, and whether the siding has the factory finish or has been repainted. If all else fails, you can match the siding color and repaint the stained parts of the siding.
Put on a pair of latex or rubber gloves. Saturate a rag with mineral spirits. Scrub at the stain, turning and rewetting the rag frequently. If it is oil-based deck stain applied within a day or so, mineral spirits should remove it.
Mix strong detergent, such as trisodium phosphate (TSP), with warm water in a plastic bucket. Proportions vary according to the type of detergent. A heaping tablespoonful of TSP added to a quart of water effectively cleans many stains.
Test the detergent on a hidden corner of the siding to make sure it doesn't remove the finish. The factory paint applied to aluminum siding is usually designed to "chalk," or gradually wear away. If the siding is older, scrubbing it may quickly wear it down to bare metal.
Scrub the stain using a soft-bristled brush and the detergent mixture. This is most likely to work if the deck stain was latex or water-based.
Clean the entire length of the affected siding boards, otherwise you risk having clean spots surrounded by grimy siding. This won't be so obvious if you clean the entire board.
Rinse the siding with a hose and cold water.