Short answer: a part designed for coating has drainage holes, hanging points and broken sharp edges, and avoids deep narrow channels, closed volumes and components that are hard to mask. These five details bear directly on coating cost and scrap rate.
1. Closed volumes without drainage
Box sections, welded frames and pockets between two sheets take pretreatment solution in and do not let it out. Heated in the oven, that liquid evaporates, escapes and blisters the film; what remains starts corrosion inside the part.
Solution: at least one drain hole at the lowest point and a vent hole at the highest. 8–10 mm diameter suits most parts.
2. Geometries that form a Faraday cage
Electrostatic powder adheres by virtue of the electric field. In a deep, narrow corner the field lines gather at the edges and the base never meets the powder. The result is visibly thin or entirely uncoated areas.
Solution: avoid channels deeper than twice their width, and radius internal corners. Where it is unavoidable, both sides should know from the start that the area will need separate work with a manual gun.
3. Sharp edges
Surface tension pulls the powder film back from a sharp edge, where it thins. Corrosion almost always starts at an edge, so the weakest point of a part is often precisely its sharpest one.
Solution: break cut and punched edges, ideally to a 0.5 mm radius. If edge coverage is a specification item, state it in the enquiry.
4. Parts with no hanging point
Every part hangs on the line somewhere, and the contact point does not get coated. If the hanging point is not set in the design, the coater has to hang the part from a visible surface.
Solution: a 6–8 mm hanging hole on a non-visible face. Where a hole is impossible, nominate an area that assembly will cover as the hanging point.
5. Areas that need masking
Threaded holes, bearing seats, earthing points and gasket faces must stay uncoated. Masking is manual work and means time per part; ten points to mask can make masking more expensive than the coating itself in series work.
Solution: mark areas to be masked on the drawing and work to reduce their number. Tapping threaded holes after coating is usually cheaper than masking them.
In summary
These five items sit with the designer, not the coater. A five-minute review at drawing stage removes a cost that would otherwise repeat on every part in series production. If you would like to review a new part for coating suitability together, send the drawing.