That's a perfect explanation, thanks. 

http://youtu.be/6N7YMr6E564?t=5m5s

They do it the same way.

This produces a scarf joint. Is this the correct description? English is obviously not my native language.

The damaged outer skin is not just sanded, it is bevelled! Ideally the bevel ratio is up to 1:40 for glass and 1:50 or more for carbon. Up to 1:100 for UD carbon.
By placing the biggest patch first the contact between the fibers of the original part and the repair patch is maximised. When cured the excess carbon is sanded away. If you placed the small carbon piece first the carbon fibers would simply butt end to end, not overlap.
Hope this makes sense.
Maybe someone else can explain this better?

That is good to see.  It doesn't explain why you go in with the big piece of carbon first as I would have instinctively gone in with the smallest first.  Can you say why it is that way round?

great video! thanks for sharing

Matthieu Libeert
Founder MAT2 Composites X Sports
website: www.mat2composites.com

How to repair a damaged sandwich part:

http://vimeo.com/moogaloop.swf?clip_id=18019742&server=vimeo.com&show_title=0&show_byline=0&show_portrait=0&color=ff9933&fullscreen=1&autoplay=1&loop=0

This video shows an approved way to repair damaged sandwich structures, e.g. aircraft wings and fuselages. Unfortunately only the repair of the outer layer is shown, but this might still be interesting for some people. If both surfaces of the sandwich are damaged the repair becomes more complicated, unfortunately I haven't found a video yet which shows how it is properly done. I'll try to post some pictures if I find them again...

Properly done this repair is as strong as the original part.

The English translation in this video is horrible, but pictures tell more than words anyway. Take note of the proper scarfing of the composite top layer, how the honeycomb is glued in with resin and resin filled with microballoons (the brown goo). What is not being mentioned explicitly but very important is using the exact same number of fabric layers as the original part, and laying it up in the exact same orientation. If in doubt the piece that is removed from the part can be burned to remove the resin and the fibers then inspected for fabric weight and orientation.

The very important part of this repair is how the patches are applied on top of the sandwich: start with the LARGEST piece of fabric, and then apply the smaller ones. The other way round will NOT result in a structurally sound repair! This might sound illogical, but that's the way it is done!

What's unfortunately missing is the final sanding and painting.