The real enemy here is not the existing silicone but any water that is in the gap. Existing silicone should be removed where possible, and the new seal should not be applied until the area is completely dry. Although silicone is totally waterproof when set, and will adhere to almost any dry surface, it will not stick at all to a wet area.
remove all silicon first, clean residue with white spirit and allow to completly dry. if the tray is plastic and subject to any movement, add weight to tray and then seal. do not remove wieght untill fully set
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