Skater lite:
This obfuscates ONLY the selected .NET file, making a backup.
Eazfuscator:
Same as skater light, not sure about the backup part...
Dotfuscator:
Duplicates the original .NET file, making the duplicate obfuscated.
So basically: No, it only obfuscates the built project.
BTW:
By .NET File I mean any .exe, dll, etc. made in a .NET compiler.