Are there differences between the matte sprays used by artists for oil painting such as grumbacher brand and those used by modelers?

Yes! The preferred modeller's spray (to me anyway ) is Testor's Dullcoat a laquer based dead flat coat - many artist finishes are alohol based and are semi permanant that is you can re- work over them they tent to go amber and I have not run acrossed a true flat in a rattle can.Testor's Dullcoat is notorious for turning shinier over time - use a fresh can and shake it well - also if you warm the can in warm water prior to use it will spark more consistantly.