Pointers on finishing cob?

I am about half way through my first cobbing experience.... laying up a 6"layer of cob around a 7" steel RMH housed within a steel bell (a furnace oil tank).  The cobbing is a lot of work (when one is working alone) but I am happy that the completed portion is hardening and drying "hard as a rock" without cracks. I'm using a mix of 1 part clay (bought for $4.00 for half a ton! which seems to be almost pure clay, a light brown color) to 3 parts fine sand, a little less than 1 part water and maybe a 3/4 part wheat stubble and pine needle mix.  As I build, I'm also adding in a lot of small rocks to make the cob go further.

My main question is , How do you finish a cob project with a smooth or even shiny finish without sealing it so it can still "breathe" (allow moisture in and out)?

I'd appreciate any pointers from experienced cobbers.  Thanks in advance for your help!

if you want to leave it as the cob look you can just sift you aggregate (sand) so that you use only fine materials. Most people make an earthen plaster to cover cob, same ingredients, finer materials. Make some samples of your clay, fine sifted sand, and sifted straw (chopped straw, goat hair, cattail fluff). Once the plaster hardens to a leather like consistency, burnish it til it shines. Use a yogurt lid or a very smooth stone. This will push the sand farther down in the plaster and bring the clay up to the surface to make it nice and shiny.

Thanks so much Daniel. I appreciate the suggestions!