In a lot of build descriptions, I've seen people refer to Mr Surfacer. What is it? What's it's purpose? Does it need to be thinned prior to use?
I have heard it is a rebottled DuPont or 3M filler primer paint. It is a grey thick paint which dries fairly fast. You can use it to fill fine cracks, or paint it on and stipple it to give a rougher texture to surfaces. If you have a rough surface you can paint it on and then sand to get a smooth surface. There are different thicknesses, 500 the thickest and 1000 and 1200 the thinnest. I'm not sure whether these numbers relate to a pigment/grain size. It can be bought in large hobby bottle sizes or in spray cans. It can be thinned with Mr Hobby Thinner (I'm not sure which ones) or cellulose/lacquer thinner. Alcohol can be used to clean off overspill. Apparently it can also be airbrushed when thin.
I find it very good for cast surface textures and for filling gaps between glued parts. It's fast drying properties help speed up the building process.