Get new ones - you don't want them coming off again in good surf

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They came off because the area on the surfboard where the pads were to be stuck was not prepared properly

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It must be absolutely clean and free of any wax, soap, polish

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You should spend a good half an hour carefully and firmly washing / wiping and washing again – boring but essential

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Scrape and use a solvent to remove any gunge from the old pads before the wash if you need to

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Absolutely clean = completely clean = perfectly free of any substance – don’t even breathe on it !

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The adhesive pad needs to stick to the surfboard surface not to the stuff you left on there

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This care in the cleaning of the surface will allow the pads to stick to the surfboard for years – the pad should wear through before ever coming off

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Of course, you need to be really cautious putting the pads on such a clean surface – they go ‘schloop’ and they cannot be repositioned !
My $0.02
