Beginner Advice: Grey artefacts when texturing a my Blender model

[smommer]

Im a beginner and have followed the Substance 'Getting Started' tutorial (Spiderbot) and decided to apply what i learned to a simple model i made in Blender. I'm getting grey artefacts when u try to apply a fill layer to my model. (see image). I did a Smart UV Unwrap in Blender, set vertex colours and checked for overlaps before exporting to Substance. I have no idea why i am getting these grey artefacts, can someone offer some advice as to why this is happening and where am i going wrong?