The ceilings in several of the rooms that were damaged from the water leak had asbestos in the ceiling texture and when it was removed, a special cleaner that was applied stained the drywall. As new drywall was added and areas were mudded again, the stain continued to carry over to everything new. After several hours of painting on primer, it still would not cover the stain. I expect it will take a great effort to cover the stain once and for all.
It reminds me of the stain of sin in our own lives. Jesus Christ is the only one who can remove the sin from our lives with His own blood that He sacrificed for us. 1 John 1:9: If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. Only Jesus can cleanse us once and for all from our sins- not just cover them up- but remove them once and for all through His righteousness.