Evil spread and background walls

[Tyro]

Can evil spread from background walls to foreground? I've noticed the corruption I planted to convert my desert once I enter hard mode has spread along background walls and now I'm not sure it's safe to kill the WoF without first knocking out the walls in my quarantine shafts. TIA!

 

[Laluzi]

No. Background walls are completely separate from blocks, and corruption/crimson/hallow will not spread from walls to blocks.

 

[Tsg TheSmartyGamer]

No other layers. Just Blocks.