Why do parts still rust after surface treatment

Why do parts still rust after surface treatment?
In the many rework cases handled by Jiantong, the common issues center on the following points:
Inadequate pre-treatment
If degreasing and rust removal are incomplete, even a thick subsequent plating or coating acts merely like a “film stuck to dirty glass”—it will inevitably peel off sooner or later.
Process selection mismatched with the operating environment
Using standard galvanizing for seaside projects;
Using simple powder coating without a primer in chemical workshops;
Failing to perform post-weld heat treatment or apply localized enhanced corrosion protection on welded parts.
Dead zones in molds and structural design
Deep-drawn or stamped parts with deep grooves or narrow slits can trap processing fluids and accumulate contaminants over time, leading to rust that spreads from the inside out.
These issues—such as fluid drainage, air venting, and the use of rounded transitions—must be addressed during the mold development stage; this is precisely where Jiantong’s in-house mold team adds value.
Improper welding and grinding
Coatings struggle to cover heat-affected zones, weld spatter, and unground areas evenly, creating starting points for corrosion.
We implement correction, grinding, and pickling (or mechanical treatment) after welding, prior to the surface treatment process.