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Has anyone used this before as the final outer render of the dome? I have been told the mix is 5:1 (lite fill:GP) but I need to know if I can use a cement mixer or not? Are there any special requirements to mixing it?
Mixing carefully by hand may be the best and will lead to the least 'compaction' of the perlite. And be careful of segregation whereby the cement tends to migrate to the bottom of the mix. Keep mixing and apply in small quantities at a time.
For insulating the dome, consider using 3 layers (76mm total) of blanket and ditch the perlite. This may be the easier option.
Didn't really answer your question, sorry. Can't help feel the perlite-cement mix might be difficult to work and to obtain a finish that is durable. We finished with 4:1:1 sand:cement:lime and with a waterproofing agent and colouring.. If you are going to paint the perlite then it might help to bind the surface better.
We did a 5:1 mix in a part of the base. It set hard enough but a lot of the cement migrated to the bottom of the layer which we discovered when trying to remove some of it. Suggest you try a small batch first.