I mask everything and spread a layer of Tacky Glue on the tender first. Then place the coal. Tamp the coal into the glue with your fingers, or a sponge. It is allowed to dry overnight. Then I use Woodland Scenic cement and an Infant Nasal Syringe to dribble glue over the entire coal pile. After it is thoroughly dry, I shoot it with a coat or two of Dullcoat. IMHO, this looks even better than anything I have seen from the major manufacturers.
Matte medium will "work", but over time it turns milky.