The only thing I would add here is the fact that some engines seem to oil and thus varnish very bad and some don't. One thing to remember is that if it gets bad enough without care it can cause heat retention. Also if you have an engine that really seems to throw a lot of fuel then you should find out why. I have seen engines loose fuel around the glow plug simply because of poor manufacturing, the same with seepage around the head. It is normal to get a little spit from the carb and of course out the pipe. Also use hi temp silicone between the muffler and head, this will help a lot and if there is even just a little mis alignment it will seal it. Like Robert Duval said in Apocalypse Now , " I love the smell of Napalm in the morning", well with us it is the obsession with Glow Fuel and the mess it leaves behind........

Have fun and happy flying...That reminds me, I have not had a dose of glow fuel in a while, I need to do something about that