I’m having a lot of fun with art students, at the moment, as we discover the art of layers.  With this particular art student, we started with wet-on-wet oil within the archway, with a layer of acrylic around the outer edge.  The acrylic was then allowed to dry with the use of a hairdryer and we applied another layer of acrylic on top of that one.  Once dry, we then covered the canvas in liquid oil, applying a final layer of wet-on-wet oil.  It can go on for ever and is a great way to pull out a certain parts of the scene that need highlighting.