Damm, he found the sore subject

Yes, you will have oil residue from the engine firewall back. I cured some of this by plugging the end of the muffle and inserting two 3/8" brass tubes inside the muffler running from the joint out the front of the muffler. This pushes the exhaust straight through the prop.. your still going to have oil from the prop slot back.

If you going to use monokote, ultra-coat or any of the film covering on the plane you want to use at least a 3/8 inch overlap of the covering in the areas of the engine and aft of the prop slot to help prevent oil seepage under the covering. it is also good to seal the covering seams with something, I was told clear finger nail polish works but I mixed up some finishing resin and applied it to the seam with an artist brush.