Declercq began his career in Ons Volkske at the age of 16, with the realistic short story "De D-558-2 Skyrocket."
He made illustrations in female magazines, like Mimosa, Rosita, Libelle, and Het Rijk der Vrouw.
Clients include Reader's Digest, Daimler Chrysler, Greenpeace, Märklin and Dai Nippon.
He had his breakthrough with the aviation series Rud Hart, that appeared in the Dutch comic magazine Eppo starting in 1977.
His watercolors won several national and international prizes (one juried by the Royal Watercolour Society, England).