Eric – Distilled water is usually difficult to come by as these days, de-ionised water is often sold (e.g. for lead-acid accumulators). If you can get it then you would only need to use ‘half a drop’ of washing up liquid, i.e. as little as possible, in say a litre of pure water. So say 1 mm cube of detergent would give 1 part in 1 million. Then there’s the problem of getting pure Cotton Wool BP to wipe the surfaces with. You would need the medical sort.