Surface preparation on metal gets less attention than it deserves, largely because the piece looks fine once the topcoat is on. The failure arrives later, at the customer’s end, and by then it is expensive.
You cannot coat over rust
Iron and cast components pick up surface rust in storage and between processes, particularly in humid months. A light bloom of rust is easy to miss under workshop lighting, and it is very easy to spray straight over.
Doing so seals the rust in with whatever moisture came with it. The corrosion continues underneath the film, and eventually it lifts the coating from below.
Clean it back properly
Two grades depending on what you are facing:
- RPC4 for light rust
- RPC2 for heavy rust
Both convert the rust into a water-soluble compound that can be lifted off, and both hold fresh rust off until your topcoat goes on, which matters if there is a gap between preparation and spraying.
The process:
- Buff the surface, or apply RPC2 or RPC4 depending on the severity.
- Apply with a cloth.
- Rub with a scotchbrite pad to lift the loosened rust away.
- Coat while the surface is still protected.
The two places coatings usually fail
Cast iron is porous, and welding joints are exposed. Both keep corroding even after they have been coated, because the coating never really sealed them in the first place.
Metal Lock is a dip treatment that forms a barrier between the metal and the oxygen in the air. Dipping components before coating addresses the two failure points that a spray gun cannot reach.
If the rust is coming from your plating line
Rust appearing in electroplated work usually has nothing to do with the coating at all. Salts left in the water carry through the plating process and cause rust to surface weeks later, which makes it very hard to trace.
Use DM, or de-mineralised, water throughout the electroplating process. Any dissolved salts will eventually show. It is worth auditing your water supply before changing anything about your finishing system.
Where plated parts keep failing
If plated components are still rusting through despite good practice, Turbo Plating makes the plating process itself rust-proof. It addresses the cause rather than adding another layer on top of it.