Two coats should be sufficient, they are laid on fairly heavy. This evening I tried some using my sandblasting gun and it worked perfect.
Been into R/C since I was about 19 years old. I think I was the first one to ever fly an R/C plane at Cam Rahn Bay AB, Vietnam
The 1/4" Volara foam has worked out fantastic, I even used it under my running boards. I tried spraying the Weldwood Formica glue but it would not do well so I just brushed it on.
I have been getting my Micro Balloons from Wicks Aircraft Supply
. They have a nice online catalog, just look under composites.
This is my bag of Micro Balloons, it is not quite a gallon any longer. You have to be careful with this stuff as it is microscopic phenolic glass spheres and they are very light. It pours a lot like a liquid and unlike a powder. It will not clump as a powder will. It floats on the slightest breeze, so wear a dust mask when using them. I have not seen an MSDS sheet on it, but I would not want to find out what it could do to your lungs.
I am mixing up some Micro Balloons and a gallon of exterior latex house paint. I had the house paint tinted a very dark gray. I had to pour out almost half of the paint in order to add the Micro Balloons. It took the entire bag to get to the consistency I was looking for. You want it thin enough to spray with a large nozzle, but thick enough so that it does not sag on vertical surfaces as you build it up. As stated earlier the bag was not quite a full gallon as I had used some. I feel confident that a full one gallon bag of Micro Balloons and about 3/4 gallon of paint is just about right.
Edited note: Do not be temped to use a power mixer as it will shatter the micro spheres and diminish the insulating qualities.