What Flagging Looks Like
Flagging is when the paper drywall tape lifts away from the joint compound underneath. You can see it most clearly under raking light. The tape edge curls up slightly, or in worse cases, an entire section pulls away from the wall in a strip.
Sometimes you can hear it before you see it. Russ noticed his when he was painting and the roller passed over a section that made a hollow sound. Pressing on the tape with his finger, he felt it depress and then spring back. That's a flagging issue waiting to become visible.
Why Tape Lifts in the First Place
Every flagging case I've dealt with came down to the same root cause: not enough mud under the tape during the original installation. When the bedding coat is too thin or doesn't fully wet the back of the paper tape, the tape never properly bonds to the joint. It looks fine at first, sometimes for years, and then humidity changes or wall flexing pulls it loose.
The contractor who did Russ's house apparently rushed the bedding coat. I scraped off a section of failed tape and the back was barely covered with mud. There were dry spots where the paper had been pressed onto bare drywall.
Other causes I've run into include using cold or improperly mixed mud, applying tape over dust or debris on the seam, and overworking the tape with the knife so all the bedding compound squeezes out before it can dry. Setting compound that's already started to harden also causes adhesion problems, which is why hot mud needs to be used quickly.
The Half Hour Fix That Wasn't
My initial plan at Russ's place was to scrape off the loose section, lay new tape over it, and feather it in. That's the half hour I quoted him.
The problem is once you start scraping, you find more loose tape. I pulled what looked like a four foot strip and another two feet came with it. Then the section above that started lifting when I pressed on it. By the time I had removed all the failing tape, I had stripped roughly twelve feet of seam.
The second problem was the existing topping coat. Russ's hallway had been painted with semi-gloss white, which I had to scrape off the seam area so the new mud would adhere. That added another hour of careful work with a 5-in-1 tool trying not to gouge the drywall.
The actual retaping went fast. But the prep took three hours. Now when I tell people I'm coming over to fix flagging, I quote them an afternoon, not a half hour.
Doing the Repair Right
The repair sequence I use now starts with full removal. Anywhere the tape is loose, pull it out. Push gently on adjacent sections to find more loose areas. Don't try to patch over partial sections, because the failure usually extends further than the visible flagging.
Once you've removed the failing tape, scrape the seam clean. Joint compound, paint, dust, anything between you and bare drywall has to come off. I use a 5-in-1 tool and a metal scraper. Take your time, because gouges in the drywall paper will create new problems later.
If you removed paint, lightly sand the surrounding area so the new mud has something to grip. Wipe down with a damp rag and let it dry completely.
For the bedding coat, mix your mud slightly thinner than usual. You want it to fully wet the back of the tape. I use all-purpose compound for this rather than topping. Apply a generous bedding layer, press the tape in firmly with a 6 inch knife, and pull through to embed it. You want some mud to squeeze out the edges but not so much that you've stripped the tape of its backing layer.
Let it dry fully. This is where I've been impatient before and paid for it. If the bedding coat isn't completely dry, your topping coats trap moisture and you get the same failure all over again. I give it 24 hours minimum.
Topping and Feathering
After the bedding coat dries, apply the first topping coat with an 8 inch knife. Feather the edges out beyond the bedding coat. Let it dry.
Second topping coat goes on with a 10 or 12 inch knife. Feather even wider. The goal is a smooth transition from the new repair to the surrounding wall so the seam disappears under paint.
Sanding between coats helps with smoothness. I use a sanding sponge for this since the paper has rougher tooth than the surrounding finished wall and a hard sanding block can leave marks. The final sanding before paint should leave no visible ridges.
Prime the entire repaired area before painting. New mud absorbs paint differently than the surrounding aged paint, so without primer you get a noticeable patch even with matching color.
When to Just Live With It
Honestly, not every flagging case needs to be fixed. If you have a small lifted edge in a closet, behind a piece of furniture, or in a low-traffic area, sometimes the best move is to leave it alone.
I have a flagged seam in my basement workshop that I noticed about three years ago and have not touched. It's behind my workbench where nobody sees it, and chasing the repair would have meant moving heavy equipment for an afternoon. Russ's hallway needed the fix because it was the first thing visitors saw when they walked in. My workshop seam can wait until I redo that wall for other reasons.
Knowing when to fix and when to ignore comes down to visibility and risk. A flagging seam will not get better on its own and will probably get worse over time. But unless it's actively crumbling or in a place where guests notice, the repair can wait until you have a Saturday to give it.