The wallpaper is truely stuck on.... really! and it has no raised pattern. So I need to flatten out the seams, and cover some picture hanging nail holes before I paint. Do I lightly sand the seams and then put a thin coat of joint compound over the seam to seal it before painting? O can I just sand the seam or just use joint compound without sanding seam.... I will prime before painting but I don't want to get moisture ( from paint) behind the wallpaper and I want a smooth surface..
thanks!!

You do not want to sand the wallpaper - it will just fuzz it up and make it harder to cover.

If any wallpaper seams or rips are raised or curled up, then cut them out with a sharp razor. Then a coat or two of drywall mud, sand and paint.

Be aware the even though your wallpaper is stuck on very well, all that can change once you paint. The moisture in paint, supported by the thick paint mixture, can take several hours to penetrate through the wallpaper and cause it to release or to wrinkle.

It's always a very bad idea to paint over wall paper and what's had to get off not will be near impossible to get off later if it's painted.
How did you try to get it off?