I find that if you boot has been used "properly" over the years or decades the only real way to get them really clean is to remove all the carpet and trim from the boot on a sunny day and give it a really good shampoo....not just with the Vax but a really good scrub with shampoo and stiff bristle brushes.....I did this on the grey boot lining (and carpet) of my 209 (most I saw were black even with grey seats...not mine!)....which looked a sort of pale brown colour with all the dirt it had picked up in 20 years. After that I left them in the sun to dry....soft, clean and grey as new!!! Could not find a boot pic!!
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