Can I seal my sculpture?

Yes you can absolutely seal your sculpture.

Wait 30 days after painting.

Why wait 30 days before sealing? 

As any cement based material cures it is using up and binding the necessary water into its matrix. If left to dry out, it will lose the needed water for a full strong cure. If too much water was used in the mix, this will naturally migrate towards the surface over the 30 days. 
Sealing or painting the sculpture too early means the extra water is trapped behind this barrier. The water will get in behind this barrier and lift it off. 
When painting before 30 days are up, we recommend painting in thin washes so the water can still migrate through this layer.  
It is not necessary to seal at all, though doing so can improve the life of your sculpture and it's' paint finish. Sealers are an additional protective layer between your work and the environment.