The final stage of insulating my brick oven was to add a layer of cement render, for rigidity and also to support the finishing layer of acrylic-based render...
From memory, I think I used pre-mix cement render bought straight from the hardware store. You could always mix your own render using 3 parts plasterer's sand to 1 part cement (and a cup or so of builder's lime, to aid plasticity), plus water of course, to bring it to a thick, creamy, workable consistency. As with the mortar, a squirt of dish washing liquid gives your render a nice creamy texture.
As you can see, I scored some grooves into the side of the cement render - this was to provide a better support for the final coat. Without something to sit on/grab hold of (especially on the sides of the dome where the surface is steep), the final coat of render could easily slide right off. To score the grooves, I just used the corner of the trowel, or my gloved finger - or probably both! It doesn't have to look great - it just has to do the job.
All told, this penultimate coat of render was about 3cm thick.
At this point (having let the render cure for a 48 hours) I actually fired my oven, just out of curiosity to see how it would fare. It held up perfectly. I noticed a wee bit of expansion as it got really hot. In this pic you can see where a small gap has opened up around where the dome adjoins the chimney...
No big deal - that's how it should behave!
Lastly, I finished the oven with a coat of pre-mix acrylic render from the hardware store. It comes in a range of colours or you can buy tints to make it practically any colour - but I chose a light natural colour, approximating sandstone.
I was in two minds about how to finish this final coat - smooth or textured. I experimented a little as I went, applying it as smooth as possible at first by hand (using a plastering trowel). I then found that rubbing it gently with a damp sponge, using large circular strokes, produced an attractive, lightly textured finish. This photo shows the difference:
Here are some detailed pics of the textured finish:
And that, my friends, was that! Here are a couple of nice pics of the finished oven, taken several years later (and still going strong!)...