Well... I do have *tried* (but failed) to read scalar QFT a few months ago. Even it is quite fail because I am completely unaware of the concept of fock spaces, I do think I have read and learnt bits that are relevant. I also still have residual knowledge of GR form my undergrad course, Ocelo7 and Johnrennie's conversations. This plus a bunch of googling and PSEing will usually help me to achieve an understanding to 1st order of something that I have little background of.
Higher order understanding will then arrive once I fill in those gaps