This weekend I managed to work a bit on the car. At first, I started to "nail down" the card strips on the rear right fender, and then apply the drywall mud. After that, I decided to apply the drywall mud in the gaps of the rear right fender, left in there the cardboard strips. This revealed to be harder than I thought,because if I applied the drywall mud carefully on the gaps, it would always make a low spot... So I decided to apply drywall mud over the entire fender, loosing all the sanding work that I have already done. At least, this way, I will have no-more that stupid strips. The day after, on Sunday, I though the drywall was already dried, but no, so I decided to mix some more drywall mud, and apply on the right side, covering all the strips done by the cardboard. After that, I had some drywall mud leftovers, so I decided to apply it to the rear upper part of the car(boot lid) and the rear part of the car. It was only enough to apply in half of the rear part, but it was a good progress. Next time, I will get the sand-block and start sanding down all that drywall mud :D 

Lesson learned: DO NOT use any more cardboard strips, apply the drywall mud directly.