acrylic on enamal?

You can spray any paint over another if it is dried thoroughly and it is applied correctly.

Even lacquer can be sprayed over enamels if you dust coat the enamel a few coats first to protect the enamel.

semibold wrote:

is is safe to paint acrylic on top of enamal?

As the others hae pointed out, it's perfectly safe.  In fact there are some situations where it is even advantageous to do so because acrylics won't dissolve and lift enamels and vice versa.  So if you're appliying a wash or even doing a lot of tedious brush painting, use a different type of paint than the base coat.  If the base is enamel, wash with acrylic or use and enamel wash on an acrylic base.    

Not a problem at all.

The usual rules apply - make sure the enamel primer is fully cured before applying your acrylic coats.