Wilky, I do not know how the sand affected the paint (as I am thinking it probably made things worse), but when you have heavily-oxidized paint, you need to do a several-step process. Here is what I would do:
1.) Wash the car.
2.) Apply rubbing compound, such as 3M Perfect-It Rubbing Compound - Fine Cut.
3.) Apple a glaze, such as 3M Perfect-It Swirl Mark Remover.
4.) Apply a really nice and shiny car wax, such as P21S, Zaino, etc.
Note that each product has its own application/use instructions and that it is important to properly use each product, so do not try to cut corners to save time. Also, you can use any rubbing compound and glaze that you want, I just recommended the 3M products because they have a very decent reputation and I have used these products on my own vehicle.
Good luck, and let us know how everything works out!