I would NOT use any lubricant in that area as it may attract more sand than it does when dry. When I lived in Florida I used a cheap synthetic bristled paint brush to dry brush the area clean after every ride and followed up with some compressed air to blow remaining dust away. You must be very careful with the compressed air as not to force the sand into the bottom bracket. If you do not have a compressor i would just follow up the brushing with a thin rag to “floss” the area. Clean, inspect and lubricate your bottom bracket and crank arms often (of course the frequency depends on how much you ride). Be careful not to force water/sand into bottom bracket when washing bicycle. I hope this helps.