The obvious answer is whenever it needs it so maybe a better question would be how often should you expect to clean a marquee.
A marquee is far more likely to get dirty when being erected/dismantled rather than when it is actually up. If you’re careful when erecting and especially when dismantling the marquee then you can usually get away with a thorough clean only once or twice a season.
There are however a few exceptions:
The dirtiest marquee I have ever had was one that was erected under some trees. When it rained all of the dust and grime was washed off the leaves down on to our lovely marquee. If you’ve got to put a marquee up under trees expect to clean it immediately afterwards.
Rain-skirts by their very nature will always get muddy in the rain, these should be given a wipe over on pretty much every job.
Traffic-film – eventually PVC can have a grey layer build up which is especially difficult to remove. It takes a long time for this to happen (a year or two normally) but at this stage it needs a thorough clean using a chemical. The material also needs irritating (gone over with a brush or similar) to get this off. Once thoroughly cleaned the PVC should be as good as new.
Tree sap is a nightmare to remove.
There are also some things that will never come out:
Petrol/Diesel can stain PVC. This will never come out so avoid them at all costs.
Ingrained mould – this affects many marquees especially those that are stored when still wet. PVC is made of many layers, if a marquee is put away wet or if cheap PVC is used then water can get inside the layers and create mould which will never come out. Incidentally this is one of the reasons we use better quality 500gsm PVC than available elsewhere, it takes far longer for the layers to break down in better quality PVC.
Some garden chemicals can stain – I was shown a marquee recently that had green stains around the rain skirt which seemed to have come from a chemical added to the lawn.
Better quality PVC always helps, we use a lacquer coated PVC as it is easier to clean and lasts longer than the cheap alternatives.
Thanks for reading