How to Remove Chemical Stains on Counter Tops
When chemicals come in contact with countertops, they first attack the protective finish and sealant that has been applied to the surface before damaging the actual counter. Since the white stains are going away when wet, it is likely that just the protective surface has been damaged.
To fix this, strip the existing finish and apply a new sealant and wax. All of these items can be found at hardware or home improvement stores and instructions for application will come with the product. Once the old sealant is removed, the white areas should be gone. The new sealant will keep the countertop protected from future stains. You could use something like this: www.kitchenzip.com/brands/Superior-Stone.html
If the countertop is large, it may be wise to have the new sealant professionally applied.