best material to be used when fairing?

So whats new then !!

A strip of stock aluminium about one metre or a little longer !!is all I use !!,when dragged over hard filler it leaves like a pencil mark !!
SO for screeding bog all I've ever used is a 1.0 or 1.2 metre steel rule !!,they are easy to use and have a Sharpe edge on all edges and even with semi hard filler acts like a oversized cabinet scrapper !!

If your glassing is up to scratch in the first place and is done properly fairing is a minor problem not a major !!
So if you need to pile on lots of fillers your workmanship and attention to detail and care and attention is in serious question .
As for spray bondo !! hi build 2 pot undercoats with Q cells thinned with acetone so it comes through a pressure pot is all I ever used !!
The acetone evaporates almost instantly AS ITS BEING SPRAYED so doesn't get trapped in the filler and if there is any left 99% of times its completely gone by the time its hardened !!
As far as I am aware Acetone doesn't react with anything I've ever used in the way of paints so its all compatible !! even using Duratek ,acetone is the only thinner I've ever used .