the resin goes outside first. mat then cloth is a little excessive (and heavy) jsut go with cloth or mat maybe two layers in some places. whatever you choose to use, it goes inside after the first, outside resin dries. when you sand, your sanding the paper, into the fiberglass. the glass gos inside to keep the original proportions outside, keep a nice finish, and keep hard lines where necessary.
personally, my dad has done fiberglassing alot throughout his 30 years in the aviation business, and he told me told me to resin inside and out. Then use fiberglass mat on the inside, and for what our purposes are, it should be enough after it dries. If you plan on making a mold like I am, he suggested to add a layer of fiberglass cloth on top of the mat for extra support.
At the moment I have about half of the inside of my helmet done, and its taken me just a few hours off and on(i can only take the smell for so long even with a mask...)
Remember, fiberglass cloth/mat sucks up resin like a good torrent sucks up bandwidth.
Fiberglassing the outside will drastically change the detail of your helmet, and create a lot more sanding work for youself.
Heres how you get around this; carefully reisn the whole helmet from the outside. After it's hardened, seperate the helmet into sections so that you can access the interior, then resin and fiberglass them from the inside.
Work in small sections at a time, and you shouldn't have any problems with warping. When it's all done, you can piece it back together.
oyu dont want to sand ALL of the paper off. just where you want to take off an edge to make it smooth your sanding will take the paper away. maybe when your prepping for paint the sanding might take some more off but not all.