We put two coats of mud in cracks of paneling, then we primed it. Then we put two coats of Ultra Behr paint and the lines are still coming through
I paint paneling frequently. The seams (lines) are more porous and tend to suck in paint and primer, causing the lines to bleed through. This is what I do for good coverage and professional look on paneling:
1) wash panels thoroughly, I use warm water with a small bit of Dawn dish soap (it cuts grease and oil).
2) use spackle on nail holes and cracks, use latex caulk on seams.
3) roll a coat of BIN 1-2-3 water base primer with a medium (1/2") nap roller, let dry 1-hour, then brush the "lines" with a second coat of the BIN.
4) roll on a coat of high quality/high solids flat interior latex paint. Apply a second coat if necessary.
I have done four jobs to date with this method and the results have been great. I do not know what you mean about "mud", are you using drywall joint compound? If so you do not need it, you should be using spackle. Always use high quality brushes and rollers, it will make all the difference.