Well, I don't know where you live, how your horse is kept, etc. But in general, sand/grass cracks are a result of excess moisture, fluctuating wet/dry (grass is dewey in the mornings but dry during the day, or frequently bathed horse put in stall with shavings, plus polished hooves that get washed off, maybe sanded) and shoes can help create them with the nail holes being the starting point.You DO mean true sand cracks, where only the surface is spliting, not the wall itself, right?
Hooves are meant to be dry more than wet. Once the little surface cracks form from excess moisture (dishwashing hands, anyone?) putting oils on there make it worse. Unlike our clean dish hands, hooves are dirty. Bacteria gets trapped by the oils and breeds, further weakening the surface of the hoof, more cracks means more oils applied...etc. Leaving them alone to dry "breathe" as much as possible and providing a good dry place to hang out most of the time or even applying a sealant to the bottom 2/3rd of the wall (when the hoof is clean and dry) can stop the cycle. Most horses do fine without the sealer if you stop applying the moisutrizers
Exercise, good nutrition and a dryer environment with regular trims does prevent a lot of problems.But right now if you live where it's been raining more than usual, you might see more of it anyways, regardless of shod/barefoot. TB's tend to have more anything that can go wrong with the hoof, they have thin walls, but that's not saying they can't have healthy feet!
Also, applying RainMaker (has petroleum) everyday is much more than would ever be necessary. About once a week is the normal application. It will certainly trap moisture in.