Does requiring specific government processes be used make this a non-commercial service?
If the aircraft itself meets the definition of a commercial item at FAR 2.101, "Commercial item" (paragraphs (1) through (4)), then painting the "commercial item" (the plane) may qualify as a commercial item, even if the painting involves a Government-unique process. This is because paragraph (5) of the Commercial Item definition at FAR 2.101 allows for some services that are provided for the support of a commercial item to themselves be considered commercial items. Paragraph (6) also provides possibilities, in that a service can qualify as commercial if it's "of a type" that is sold competitively in the marketplace.
Open full Question Details
Ultimately it's the contracting officer's determination, using his or her knowledge of the painting requirement and the commercial item definition in the FAR, whether the painting requirement can be considered as a commercial item.