Bad emulsion put on walls

Sand down the area using fine grit sandpaper. Thoroughly clean and dry the wall, and apply a coat of primer if necessary. Repaint the surface using a paint brush (or a paint roller for larger areas). Apply the emulsion thinly and wait for it to dry before adding a second coat

I will prepare the wall again for a new paint to go over it I will sand and the bad wall and painter again

It depends how bad the emulsion was and did it really need to sand it down noraly if it was just the wrong colour you would just put the right one on but if it was bad with bits in it scap the bit of sand it repaint the wall

I will prepare again the wall for painting going throw all the steps :sand ,primer,paint 2 coats