I picked up a packet of citric acid from a local Asian supermarket, but apparently home brew shops sell it too. I used a very large garden seed tray, big enough to take a window and pored in the acid granules mixed with hot water, then left overnight in the garage.
These are before and after pics, the first window I did was in worse condition than the second (I forgot to take a photo) but has come up the best. I'm really impressed with the results, a good polish and they should be good to go...




