Can I seal my sculpture?

Yes you can absolutely seal your sculpture.

Wait 28 days after painting.

Why wait 28 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. 

A sealant such as Modern Masters Masterclear supreme in matte finish, (a specialized outdoor polyurethane) post 28 day cure, may help to seal in the color and protect as it has added UV Blockers and Absorbers that greatly reduce the sun’s graying and fading effects on stained wood and painted surfaces.