Hi! Checked the UVs after applying the modifier as it was setup in the file. The UV borders of holes remain square, which I think explains the artifacts your getting: This is due to the subdiv modifiers UV smooth setting is set to "Keep Boundaries": You can preview modified UVs without applying modifiers by checking…
I tried applying Polypaint before I create the IMM Brush/Object, But when I try to use the IMM Object Brush it does not apply the Polypaint. You mean applying the Polypaint before 'create InsertMesh'?
There are lots of technical things we could mention but you've made a little truck and it looks like a truck so you made a good start. I think you should learn to apply UV mapping and paint some nice textures for it next - that covers the basic requirements