Sitting outside the insulation, this layer serves to help mould the desired shape of the oven (in conjunction with the insulation). This is usually a high strength mortar and, if your oven is exposed to the elements, we recommend the use of a waterproof mortar to protect the insulation and inner dome. This is especially important if you don't intend on painting the outside decorative layers.
In our higher end kits we supply specialist waterproofing mortar with additives to to improve its performance and ease of placement. This is especially important for the novice builder.
There is also a plastic trowel in the kit which aids in material workability and to achieve the desired curves.
