If you are speaking of very old wallpaper that is in fact paper, you might get away with it.
The difficulty come with the adhesive used to put the wallpaper up... most of it is water based, which means when you saturate the paper the adhesive lets go, and the whole thing has the potential to bubble, wrinkle, or in some cases fall right off the wall.
If the paper is newer, it may be vinyl or vinyl coated, in which case the mud won't adhere well at all.
My best suggestion is to strip off the paper, before applying the mud.
It shouldn't be a problem. The glue is water based and the mud should adhere to it.
you can put it over but the wall will look a bit fat
That's an indication of water damage. It needs to be replaced.
It's best to remove wallpaper (as best you can) before repainting a wall. You can rent a wallpaper steamer at your local tool rental place (U-Haul, etc...).
Read the paint MFG recommendations for full cure, or 48hrs.
It shouldn't be a problem. The glue is water based and the mud should adhere to it.
Usually, there is drywall under the wallpaper. It is not difficult to remove the wallpaper (by steaming, or another method) to get to the basic drywall. Then, you can texture it and then paint it.
To remove superglue from sheetrock walls, you can try using acetone or nail polish remover. Soak a cotton ball in acetone and gently dab it on the affected area, then wipe away the softened superglue. Be careful not to damage the paint or finish of the sheetrock while doing this.
you can put it over but the wall will look a bit fat
You cannot use sheetrock as flooring, period. If you meant on a wall, there is no reason you cannot put sheetrock over tiles. The question is why would you want to! Why not remove tiles and material behind, which is probably sheetrock, and then install new sheetrock. If it is an area prone to moisture you could use moisture resistant sheetrock. The job would look better and you would not lose the space by adding over existing material.
can you just wallpaper over paneling and not do any thing else if it has small groves
Putting plastic over the stud walls before sheetrock is not recommended. The moisture will be trapped between the sheetrock and plastic and the home will not breathe.
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It is put behind a fireplace as a precaution, like waterproof sheetrock in bathrooms.
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That's an indication of water damage. It needs to be replaced.
It's best to remove wallpaper (as best you can) before repainting a wall. You can rent a wallpaper steamer at your local tool rental place (U-Haul, etc...).