There are so many options for finishing the outside of the oven but it is usually easiest to use a cement type mortar to smooth the outside and give it that professional look.
We highly recommend using a professional plastering ‘system’ rather than an unreliable hand made mix and applying multiple coats over a few days rather than trying to plaster the outside in one step.
We supply a professional three part plastering system with our our higher end kit sets. It may seem extreme to have three different coats, but the result you achieve is well worth it. There are very detailed instructions on how to mix and apply the plaster.