Someone recently posted the question of how to clean the sludge staining from inside the radiator overflow bottle. This has always been difficult or impossible but I think I just discovered a good solution. Last night I removed the bottle and filled it with a 20:1 mixture of Marine Clean; the cleaning prep product for POR-15. I let it soak overnight and in the morning I simply rinsed it out and voila! It now looks like a new one! May not work on T2 kombis. Individual results may vary.