We could all also take a moment to remember that only a very short time previous to the events shown in the picture, Sanguinius killed a bloodthirster in some epic battle, and was also involved in a _very_ prolonged battle (weeks) outside of the Emperor's palace with entire legions of chaos space marines.......
Are we really going to question a little bit of battle damage on his armour?
PS: The sword on the ground belongs to Sanguinius. It is featured in many pictures of him. It is always red, too.
All of this has happened before.