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Dual-action, as Iwata describes it

The official NEO CN / BCN operating manual walks through fine-line spraying in three steps: “Depress the main lever to start air flow.” Position the airbrush close to the surface. “Pull the main lever back slightly to start the flow of paint.” Line thickness is then controlled by distance and by “varying the paint flow by manipulating the main lever’s position.” For wider coverage, the same manual says to depress the lever, move farther back, and pull the lever back farther to release more paint.

Every Iwata and Master model in the beginner shortlist is officially listed as dual-action (and gravity-feed). Iwata’s Eclipse HP-CS and NEO CN technical tables both have the field “Action: Dual.”

Single-action, as Paasche describes it

Paasche’s official listing for the H Single Action (H-1AS) calls it a “Single Action - External Mix - Siphon Feed Airbrush.” The copy says the assembled size #3 head can produce “fine lines as narrow as 1/16 inch and spray patterns up to 1-1/2 inches wide,” and that “its simple, user-friendly design requires minimal practice compared to double action.” The related H#1L listing likewise describes a “single action, external mix, siphon feed model” with a chrome-plated body, marketed for hobby and craft work among other uses.

Paasche’s 2000SI listing describes a “single-action, internal mix airbrush” that can produce “precise lines as narrow as 1/32 inch and spray patterns up to 1-1/2 inches,” and says it is “ideal for solid coats, uniform lines, and stencil work.”

What those official descriptions imply for Gunpla

• Dual-action, in Iwata’s manual, is a two-part trigger: air first, then paint. That is the control method the beginner gravity brushes on this site are built around.
• Single-action, in Paasche’s listings, is presented as simpler to learn and well suited to uniform coats, lines, and stencil work. Those pages do not describe independent paint-vs-air fingering.
• Feed style is a separate official field. NEO CN and Eclipse HP-CS are gravity-feed; the Paasche H examples above are siphon-feed. Do not treat “single-action” and “siphon-feed” as the same thing — Paasche also sells other configurations, and we only quote the listings we opened.

If you are choosing a first Gunpla brush, start with the beginner guide. For tip sizes, see the needle-size guide.