Flushing with fresh water is usually enough. Avoid the use of lubricants as these tend to attract sand and result in jamming. Plastic boats and the decks of most folding boats usually contain UV inhibitors which will prolong the life of your kayak for years. Scratches and the inevitable scars of memorable adventures are normal. If however, a gelcoat scratch is especially deep and you can see the hairs of the fibers sticking up, then it is time for a small gelcoat repair.
Kits are available.
Use duct tape as a temporary measure. Plastic boats can only be repaired by the manufacturer. Folding boats need special care and should never be dragged on the beach. Sharp rocks and barnacles will destroy the hypalon hulls. Sand is particularly destructive to joints and ribs.