I hung some vinyl wallpaper in march this year 2018. Recently client noticed some rust coloured stains have appeared behind the sofa only. Also some darker black looking patches under the paper. It was paste-the-wall paper. Walls were stripped, prepped and then painted white with dulux supermatt before hanging the new vinyl paper. Any ideas as to the cause. I've asked client to monitor for a couple more months to see if staining appears elsewhere in the room. So far only behind the sofa
Walls weren't a consistent colour, new paper was white. I've always used supermatt for its good opacity.
I would never paper over supermatt without sealing it first, Zinsser Bullseye would have been a far better choice, I always cross line walls under vinyl wallpaper as it gives a superior finish. It appears that something on that wall has been activated by the paste probably due to lack of air flow. You will have to eat humble pie and start again on that wall, strip off the paper and wash the wall with a hot sugar soap you will almost certainly find the Supermatt washes off as well, treat the wall with a fungicidal wash and let dry overnight rinsing off the following day. Bullseye the wall, cross line, let dry and hang the vinyl the following day.
Regular paste the paper wallpaper, yes I would have used bullseye but as it was PTW. Wondering if it could be sofa being put right back against the wall before paste had dried completely?
PTW is not a problem over Bullseye. As suggested in post #4 air flow may have been a contributing factor. If it was a job I had done I would come to a compromise with the customer and offer to re-decorate that wall for free on the understanding they supply the materials.