What is the purpose of UIC Leaflet 842-3?

The purpose of UIC 842-3 is to define the technical requirements for preparing metallic and non-metallic surfaces on railway vehicles. It ensures surfaces are correctly cleaned and roughened to guarantee the long-term adhesion of paint and corrosion protection systems.

What is Sa 2.5 in railway painting?

Sa 2.5 is a standard of surface cleanliness defined in ISO 8501 and referenced in UIC 842-3. It stands for 'Very Thorough Blast Cleaning,' where the metal surface is 95% clean of all rust, mill scale, and contaminants, appearing nearly white.

Why is degreasing important before shot blasting?

If a surface is not degreased before shot blasting, the abrasive media will spread the oil and grease across the entire surface and drive it into the metal pores. This causes poor paint adhesion and premature coating failure.

The Foundation of Durability: UIC 842-3 and Surface Preparation

Discover UIC Leaflet 842-3, the essential standard for surface preparation in railway vehicles.[1][2][3] Learn why proper cleaning and shot blasting are the secrets to long-lasting corrosion protection.

October 15, 2023 11:42 am

What is UIC Leaflet 842-3?

UIC Leaflet 842-3 is a technical specification titled “Technical specification for the surface preparation of metallic and non-metallic materials used in the construction of railway vehicles and containers.” It establishes the mandatory procedures for cleaning and treating surfaces before any paint or anti-corrosion coating is applied.

In the railway industry, the longevity of a vehicle’s bodywork depends less on the quality of the paint and more on the quality of the surface preparation. Even the most expensive epoxy coating will fail if applied to a surface containing rust, oil, or mill scale. UIC 842-3 defines the mechanical and chemical processes required to ensure perfect adhesion, acting as a prerequisite for the painting standards defined in **UIC 842-4** and **UIC 842-6**.

Key Processes: From Degreasing to Blasting

The leaflet outlines a strict sequence of operations to achieve a “chemically clean” and “mechanically keyed” surface:

Degreasing: Before any mechanical work, all traces of oil, grease, and drilling fluids must be removed using solvents or alkaline cleaners. Blasting a greasy surface simply drives the oil deeper into the metal.
Mechanical Cleaning (Blasting): The standard typically mandates abrasive blasting (shot blasting or grit blasting) to remove rust and mill scale. Ideally, this is done in automated booths.
Roughness Profile: The blasting process must not only clean the metal but also create a specific surface roughness (profile). This microscopic “texture” allows the primer paint to anchor physically to the metal.

The “Sa 2.5” Standard

UIC 842-3 frequently aligns with **ISO 8501** regarding visual cleanliness grades. The industry standard for railway rolling stock is typically **Sa 2.5 (Very Thorough Blast Cleaning)**.

Sa 2.5 Defined: When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint coatings and foreign matter. Any remaining traces shall be visible only as slight stains in the form of spots or stripes.

Comparison: Preparation Methods

Feature	Manual Cleaning (Wire Brushing)	Abrasive Blasting (UIC 842-3 / Sa 2.5)
Cleanliness	Removes loose rust only (St 2 / St 3).	Removes all rust and mill scale (Near White Metal).
Adhesion Quality	Low (Paint may flake off).	High (Paint anchors to roughness profile).
Suitability	Minor patch repairs.	New build wagons, heavy refurbishment.
Contaminants	Often spreads oil/grease.	Removes surface contaminants (if degreased first).

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