"Placing blocks on a not repaired scrap station will turn any block into scrap, so you will lose that block, unless you buy the station. Only newly generated Station are scrap station, so if you have build on them already, everything is like it was before on those."
I also have another idea. It's pretty irrationall that if You place block at decayed block then it changes into decayed also. Let's make them infected instead and make overlays full of wild vines and cracks so it will be more reasonable.