I DID use GW Matt White spray as an undercoat on this (and the other El Viejo Dragon) model, but I don't usually. Normally I use a slightly thinned down Humbrol matt white which I apply with a brush. I think some of the coarse look of the pics may be down to the fact that I haven't got a very good camera so I sharpened the images - this tends to make them look 'bitty'. I'll bear your advice in mind tho, thanks.
I've noticed onthis and another image you've posted a slight touch of roughness in thepaint. After due consideration I'm thinking that this might be due to the primer being too coarse. (GW's White can come out like this) you might want to either move the figure further away from the can 10-12 inches (30 CM) or change brands. Other than that your work looks pretty good and with some use of glazes or tonal alterations you'll be stepping up.