I store mine at Lowdham leisureworld and I pay them once a year to wash it. They fetch it off storage and return it once its done. worth it as I have nowhere to wash it.
If you haven't noticed a 'jet wash' doesn't have to do as it says on the tin ... in other words you can use the nice hot soapy brush to do the cleaning then use the spot free water nozzle to rinse off with ... voila ... nice clean van without blasting any sealant out of anything ... just make sure the forecourt is big enough to drive through & out of though
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