Concur - - NOT the spray foam. I use a 4lb foam from either SilPak or Burman. I like the Burman foam, but like the container the Silpak comes in. You can't go wrong with either.
As far as customizing the skin. If it's big ALL OVER.... foam it - then tailor specific areas. When foaming, do it is SMALL portions and small areas.... a little at a time. Because the foam is such an adhesive, it sticks to itself with no issue of separation. So, TAKE YOUR TIME. the skins are much easier to foam while they are in the molds, but can be done outside the mold if you TAKE YOUR TIME.... do small areas at a time.... and SMALL portions are you go. OH = did I say that already???? ok good the point is made.
The A & B are measure in a ratio - I'm nto at home at the moment, but I think it's like a 33A to 66B.... but the container if VERY clear as to the ratio. buy yourself a digital scale and measure. Don't try to eyeball this measurement. To do SMALL portions at a time, jsut do the math.
33-66
16.5 - 33
8.25 - 16.5
you get the idea. I measure weight in grams. You don't HAVE to be exact to the .5 gram, but if you want the true density of the foam.......