As we never get very hot in the UK, I thought I'd ask some advice on this forum about this question:
- In the hot sun here (30's C / 90's F in the shade) is it better to dry out my thermal layer before adding the insulation layer or is it better to add the insulation layer straight away to protect the thermal layer from the sun and slow down the drying? The first test brick I made was pretty hard within a few hours, though it was only 1" thick.
Today I will make some test bricks with different clay/sand ratios. It was hard enough for me to get any kind of clay so I don't really want to trying to hunt around for a different type/mix.
First thing, your link doesn't work as is with the last parenthesis, it thinks an 8 followed by a ) is a smiley. If you make sure the URL ends with <<thread=1498>>, it works.
From the picture at your link, it looks like you need some more fiber in your cob mix. If it seems to be drying too quick and cracking, this will help out. Straw, dried grass, even shredded paper can be used. As long as your mix has tensile strength to it (from the fiber) it can dry quickly in the hot climate and still be strong.
A lime plaster will definitely make it more rain resistant. Here is a link to a page on historical preservation of tabby, a lime-seashell-sand mixture. It has recipes for stuccos that can be used protect tabby and similar mixtures can be used to weatherproof your cob.