what exacly does this stuff do ???

I've never used those products, but if you go over to this page on Swanny's Models, it goes over them fairly well.

Hope it helps! I use that site a lot myself!

Mr. Surfacer is basically like thinned putty.  You can spray it to fill in sanding scratches on a large area, use a small brush to precisely fill in minor seams.  You can also texture the stuff to simulate cast iron, non-skid surfaces and what not.  You thin it with lacquer thinner.  I don't use it a whole lot, but having a jar around can be mighty handy for those types of situations.

Andy

I've used Mr. Surfacer as a primer coat thinned 1:1 with laquer thinner with pretty good results.  It seems to provide a really good surface for accepting the acrylic paint I use.

TomB 

Thanks all of you.The Swanny's  article was particular inciteful.  Semper fi,  rudedog

save for massive gaps I use all three -500,1000,1200 almost exclusinvely as my filler- primers - its brushability gives un-parralled control in  tight spots - build it up in layers for larger areas - you'll find yourself sanding allot less . It dries  quik,  is not brittle can be scribed what more can you ask for (just don't use the 500 as a primer!)