Painting metal over the white of the foambord can be a demanding situation, so it is always better to apply a base layer of some dark colour before, I do not recommend the use of sprays since most of them will tend to melt the foam of your foamboard, so an acrylic paint and a brush it's always a better option. Also despite taking more time you will spend les money since the sprays tend to run out very quickly.
A closeup of the bases before glueing the models: the Hashut rune in the left one disappeared after painting the black! so i had to improvise and paint some magma in the empty spaces left by the sand
A good thing of combining metal and fire is that the red glow of the lava is automatically reflected by the metalic paint as can be observed in the foot of the left model.