Any information on this would be greatly appreciated.
It may be time to have a little round-up of what we know (or think
We know that his pen of choice (at least for some part of his career) was a dip-pen, fitted with a Joseph Gillott Inqueduct G2 nib. It took him some time to come up with the ideal combination, and it doesn't seem certain when he did so.
He usually worked on art-board, at about A3 in size, seemingly cut from larger sheets.
Initially he appears to have penciled directly onto the art-board, and then inked that - this as much a time-saving device as anything; while he did apparently do preparatory work for Tintin, and tried to work ahead, due to the pressures of doing that and editing the rest of Le Petit Vingtième
, he also was known to do his two pages of Quick & Flupke
in a couple of hours the afternoon of the day that they were due.
Later on, after the war, he seems to have moved on to a more structured system, producing thumbnails and roughs in pencil, felt-pen or biro, often on plain office stationery, from which he worked up more complete roughs, refining as he went.
He used a large library of cuttings he kept and maintained himself (the only other person allowed to work on the archive was his father) for reference. Additionally he himself, sometimes with other studio members, or, at others studio members on his direction, would go to a location to photograph and sketch material as specific reference - old concrete war-time coastal bunkers for the gun-emplacements in Flight 714
, for example, and probably most famously, Bob De Moor's trip to the U.K. to update The Black Island
Hope this goes some way to answering your questions - I'm sure there is more to be said, as it's a big subject!