They’re pouring latex on him to make a false chest.
2So that they can place the arc reactor prop in him and make it legitimately look like it’s embedded in his flesh.
4The latex is colored so they can see where they are applying and how thick the layers are.
5It will then be airbrushed to his skintone and details like nipples scar tissue discoloration will be added.
6Then the reactor prop is added to the dimple. Basically the latex becomes fake skin and they tear part of the center open.