Well you were exposing for the sanded, which probably means everything is going to be underexposed. You should expose off some rock or something grey directly under the lights. The sand is not something I would use. Of course you can meter off the sand, figure out how wrong it is, and then use that figure repeatedly.
If you meter off something bright, your picture will be too dark and you have to back off. If you meter off something dark, your picture will be too bright and you have to up the exposure. If you meter of something right in the middle, you may get a new girlfriend named Goldilocks and have 7 kids.
Now if you WANT the image to be too dark or too bright that is your creative right, I am just saying as a general rule.