More Quentin Blake

I've been rabbiting on in the post below about Quentin Blake.
Going through his website I found an interview and a diagram showing how he completes his work.
In brief, he draws his image, pops in onto a lightbox then traces it onto watercolour paper.
Perhaps it's because I prefer to use great whopping thick pieces of paper, but I find it surprising that he's able to see anything through it.
Am I being completely naive here or is this a common enough thing?