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There are industrial sprayers that spew proportioned fiberglass ingredients, but it does not do much to protect insulation from heat. Only high-temp insulation can protect low-temp insulation. Airflow and reflective surfaces help, too.
To learn to use mud, you can practise on walls or pottery. The best additive for increasing workability is fumed silica, or cab-o-sil. It makes the mix thixotropic, like catsup, so it stays where you put it. Microballoons make it easy to sand, fibers make it stronger. I usually use all 3 in some proportion. Don't try to get it perfect right away. Start by filling low spots, with the mistakes seldom above the finished surface. They are easy to knock off while half-cured, and provide a reference for further trowel work. You can also cover an area of filler with clear plastic, and work it that way, peeling the plastic later.
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