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