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On some light fixtures, the base is barely larger than the electrical box or its mounting plate, so it won’t cover any imperfections in the wall, and it’s essential that the wallpaper comes up exactly to the very edges of the mounting plate. I often remove that mounting plate so the paper can go under it, which gives a neat look.
In this room, the light was changed from one fixture centered over the sink to two wall sconces. The electrician had a hard time fitting the new boxes into the wall. (It is much easier on new construction.)
There are a lot of things going on wrong with these sconce settings, but some are not visible and are difficult to explain. It took me about an hour to figure out what was going on, and how to rectify a box that was cattywhompus in the wall – but that’s a different story.
Here you see a gap because the sconce base is too small to cover the hole for the electrical junction box. This fixture had a larger (3/4″) gap on the other side that is not pictured. In the next photo, the box is extra large, and extends out beyond the small sconce base.
I had to cover up those gaps to make a solid base for the wallpaper to hold on to. In the case of the blue box, I had to smooth over the ridge caused by the thickness of the blue plastic against the wall (to prevent a ring from showing under the wallpaper, all around the fixture).
To bridge the gaps, I used a certain kind of paper, dunked in Gardz, a penetrating wall sealer that dries hard. That essentially recreated the portion of wall that had been cut away. Once that dried, I skim-floated over it with joint compound and then sanded smooth, to even everything out.
I used joint compound again to float all around the ridge on the blue box, and got a perfectly smooth wall.
Since I had been able to remove the mounting plate, I was able to get the wallpaper to fit under it, so no gaps showed around the base. Then I reconnected the wires and rehung the sconces.
As you can see in the finished photo, it turned out great.