For giving the rib the right heigh we made this tool that sands one rib and supports on the other 3 ribs.But also the sanding paper has an heigh, so we used the same sanding paper reversed.It's necessary to follow the sanding lines as shown in the following picture.

 

It is very important to sand following the sanding lines because the profile of the rib varies depending on the rib's lenght.

After the first rough work we used this long straight ruler to sand the ribs.

 

 

The Ruler is used also while checking for right alignement.Also in this operation you need to follow the lines as shown before.