Smoothing old walls (painted wallboard not plaster)

I have a number of walls where the joint tape is still a visible bump on the walls under many layers of paint. What sort of preparation of the surface would be required before additional joint compound could be applied to try to smooth these walls? Thanks!
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If there's a linear bubble under the tape, remove it and start from the beginning.
If it's sound, degrease (e.g. with tsp) and hand-sand with 60-80 grit or whatever you have that's close, wherever compound will be applied. Apply however many THIN coats of compound it takes to hide the joint, with the usual progression from 4" to 12" knives. Joint compound will stick to just about anything; this will get it to stick really well.
J
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In the past 2 months, I used a carbide 2.5" paint scraper to remove the many layers of built up paint and the crappy blobs of texture from all vertical walls in my 1973 built POS.
This process left many divots where the texture pulled off the wall, but by using 2 coats of thinned mud, after sanding the wall was perfectly smooth. In addition, like yours, the tapped seams were terrible, but when I was finished they were nearly invisible.
IF you scrape your walls, you don't want to go through to the drywall surface, just knock off the high spots, so when you apply new mud, it goes on smoothly. Something that really helped me was thin mud and using a broad knife. When applying the second coat, spray the wall lightly with a mist of water after getting the mud relatively smooth. It's impressive how smooth the mud turns out, ultimately it needs just a touch from the sander. I found it easier to scrape and smooth with mud than applying new mud over the top with out scraping which took something like 4 coats. Don't know what your situation is...but I at the same time I scraped the acoustical texturing off the ceiling (glad that crap is gone!).
One wall in particular had many bubbles under the tape, and what I learned was the original tappers filed massive cracks with mud, which later on, pulled away from the tape. If you have to remove the tape, take a knife, cut along each edge of the tape, through the mud and paint. With a putty knife and a spray bottle, start at one end and lift the tap out of the joint. Spraying a small amount of water on the tape really helps it to let go of the mud. Repair the mud in the joint with a narrow knife (I used a putty knife) making sure to keep the edges of the channel relatively clean. Lay new tape back in the track with fresh mud, feather the edges you'll be good to finish like it was new. Again...if there's massive amounts of mud on the seam...the paint scraper works well to level out large bumps, surforms work well too.
When you get the walls like you want, grab the texture gun and get to work. Granted this is a lot of work and wouldn't wish it on anyone...but the house looks fresh and renewed, and I'm glad I did it.
Good luck...
Darwin
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Just out of curiosity, if you knew then what you knew now, wouldn't you have just torn out the drywall and replaced it? It seems to me that the work to recover from a crappy drywall job exceeds or at least is as much as the work to just tear it out and do it over correctly.
Obviously the materials cost might be higher, but ignoring that,
Ted
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That's a fair question, and I considered it mostly I didn't want to deal with the insulation from the ciling. While scraping a room say 12 X 14 took only a couple of hours, each coat of mud a couple as well. So the while the details sound like a lot of extra time/effort it wasn't as bad as it sounds.
Darwin
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