I’m just a hobbyist and these are my experiences…
My modeling nemesis for a very long time was obtaining nice dead flat finishes. The main problem was getting a consistently flat finish while avoiding a milky or dusty appearance – oftentimes the stuff that’s in the varnish that makes it go flat also alters the painted appearance.
After trying practically all the scale model paint varnishes, I have found the following to consistently give the best flat finish without a “dusty” appearance.
TAMIYA TS-80 Flat Clear, is a lacquer spray similar to the old Testors Dullcote. It provides a clear dead flat finish. Since it is in a rattle can, it is easy to use. The spray nozzle is quite good. It is forgiving if it is sprayed a bit heavily by accident – just turn that surface up horizontally and let dry; it self-levels. However, a very excessive amount on top of a matt acrylic paint job can cause cracking of that base coat. It dries quickly. Many light coats may be necessary to completely make a gloss finish disappear.
I also like to spray this on scenery; all of my layout is covered with TS-80 (even the structures). It is best to apply it in light passes; in the past a heavy coat had turned an area of dark cinders to light grey. I find that spraying this on grass tufts takes away the unrealistic sheen they have out of the box.
I once applied a very little of Nano 5W Oil to locomotive axle journals (in order to wick 10W lube) and over time it wandered right up the boiler! A thin spray of TS-80 “absorbed” the oil and restored the flat finish.
Stripping it is more difficult on styrene models – I believe that 90% rubbing alcohol would work. On a brass model, lacquer thinner makes short work of removing TS-80 and any other paint on it. But, TS-80 is a bit expensive and I only use it on rolling stock these days.
A more cost effective flat clear is Winsor & Newton Galeria Matt Varnish. The bottle must say MATT VARNISH, not MATT MEDIUM which they also offer (this is mixed into paint). It is an acrylic that can be airbrushed.
It must be shaken prior to use, even though I did not find any instructions on the bottle to do so (I found out by not doing this, the application went gloss). I use distilled water to thin it, but sometimes shoot it full strength. It provides very consistently dead flat very clear finishes. In my area, it is not readily available and I have had to order it online from the United Kingdom.