Until the new boats came out, the keels for the Santana 20 were made by pouring molten lead into a fiberglas shell that was immersed in water. The heat from the lead and the cooling from the water caused the fiberglas to change shape. So Santanas typically have waves and hollows in their keels. This is not as big a deal as you might think. So long as the high spots are sanded down and the low spots are filled in, the boat will be fast.
Problems start if water gets in bewtween the glass and the lead.
This can lead to the keel deforming over time. If you live in a cold climate and the water trapped in the keel will freeze, then the keel that measured in beautifully one year can be way out of spec the next. If you get to a hollow sounding place, drill into it and see if any water comes out. If not, fill in the hole and go sail. If water comes out, you have a problem.