I accidentally dropped mine onto the beach and of course got sand onto it Thing I did was first of all before I folded the phone Got a bottle of water to gently trickle water over the phone and got as much off as possible. Then use tissue paper to dry water out of the hinge area. Water getting drawn out the sand comes with it. Some very fine particles still got in. Was a touch crunchy folding it for a few times, but it's all good now.
Thing is to clean it before folding it. If you didn't, once the sand gets in. You have no way of cleaning it out