The screen washer tank on my car is a semi transparent white polypropylene type container. Over the years a dark algae scale like deposit has formed on the inside surfaces, mostly above the water line. I removed the container from the car thinking it would be fairly easy to clean out, but the deposit has defeated all I have thrown at it.
I've tried descaler thinking it to be a limescale build up from tap water, hot washing soda that shifts the stains in my teapot, and lastly, with my Marigolds on, caustic soda. Using a stiff brush I've managed to clean up the surfaces that can be reached through the filler hole, but I think this is more by force than any effect of the chemicals used. Very dark patches persist in the areas, nooks and crannies out of reach.
Any ideas as to what the growth is and how it might be dissolved from the inaccessible areas - apart from buying a jar of 'screen washer container scale dissolver' at the local car accessory shop?
Roger