Removing wallpaper on concrete wall and prepare for paint

Question

I have removed the textured wallpaper from the wall and the concrete is now exposed. There was a significant amount of glue, and I have used warm water with sugar soap and a scraper to remove as much of it as possible (the process was repeated 3-4 times and little glue was removed on the last pass, but the wall still feels very slightly sticky when wet).

Small holes have been patched with setting compound and then sanded. The wall is now fairly smooth, with some irregularities.

I am looking for a smooth (but not necessarily perfect) wall and long-lasting paint job so the glue is a concern. I am thinking about proceeding as follows:

• Apply a layer of sealer to the wall (Zinsser Gardz High Performance Sealer, a water-based product. It looks like oil-based sealers were popular in the past but because of their smell and thanks to technical progress water-based sealers are now a good option.)
• Skim coat the wall with some gypsum-based joint and filler compound, then sand (repeating the process twice).

Does it make sense to process in this order or should skimming happen first? Would this method yield good results?

I'm aware skimming can be tricky but I'm happy to learn how to do it. I have considered putting wallpaper on the wall but would prefer not to if there are decent alternatives.

Answer 1

I think you have the right idea. Wall paper is a huge pain to remove correctly, and that glue will absolutely cause problems with future coatings if it's not dealt with.

I have 2 suggestions.

1. If you can muster the willpower to clean the walls one more time, get some TSP (trisodium phosphate, the real stuff, not the "TSP Substitute") and use it mixed with hot water to really scrub, then rinse with water. It does a good job of eating up that gunk.
2. Use an oil primer. As good a product as Gardz is, you absolutely must avoid any kind of water based product being directly in contact with the remaining paste residue. The water can re-activate it and cause poor adhesion and peeling. If that base coat doesn't stick, anything else that goes on top of it will fail sooner or later. Get a respirator with an organic vapor cartridge, a can of mineral spirits... and maybe a couple of fans. It's kind of a pain, but it dries fast. It's really not that bad.

Also, @JACK makes a good suggestion in the comments to deal with the texturing, but I will add a warning: don't mud directly on top of the unprimed wall, for the same reason I recommended not using a water based primer.