This discussion is a continuation of this previous thread:
https://board.moparts.org/ubbthreads/ubbt...tml#Post2482406I decided to write about the special case of rusting caused by mill scale, because I haven’t seen it discussed here before. The problem is well known by fabricators and owners of large steel structures, as all hot-rolled structural steel has mill scale. Cars are mostly made of cold rolled steel without mill scale.
A few forged or hot worked parts, namely the bumper supports and the anti-sway bar, had paint applied by the factory over the mill scale that resulted from the hot working. Factory paint over mill scale presents a problem for restoration of possible ongoing rusting that should be understood and fixed, or the parts may keep on rusting even under new paint.
The bumper supports and anti-sway bar on my car had more rust damage than other parts of the car, not counting body panel perforations in the usual places. I believe mill scale was the culprit.
Mill scale is an iron oxide called “black rust” that is not as destructive as red rust. However, this black rust is often unstable and prone to change to red rust. When paint is applied over existing mill scale, in time the mill scale may change to red rust, causing rusting under the paint. As long as any mill scale remains, the chance of red rusting continues.
One recommended way to remove all the mill scale is by “white” blasting. The term “white” denotes the appearance of blasted steel when totally all black rust has been removed down to bare metal. But some restorers prefer to de-rust with Evapo-Rust rather than blasting. I have a small blast cabinet, but I’d rather use chemical de-rusting.
Mill scale will not be removed by Evapo-Rust except by extended submersion and some physical removal. Some effective means of physical removal are by powered wire brush, wet sanding, and “pecking” at thick deposits with a hand held chisel (without a hammer). Extra thick deposits of mill scale may happen when loose flakes of mill scale are pushed or pounded into hot metal by rolls or hammers. This condition is called “rolled-in mill scale” or “scabs” and requires special physical treatment to remove when using Evapo-Rust.
The photo with red background at the top of the other thread shows a part that was dipped in Evapo-Rust for a short time, probably a half hour to a few hours. The part is still black with mill scale after cleaning with mineral spirits. Evapo-Rust coats parts with a black carbon solution that should not be confused with black rust or mill scale. The black solution is readily removed by applying mineral spirits with a brush and wiping with a rag.
In this photo the part has been in Evapo-Rust for two days and still has mill scale as evidenced by blackness remaining after cleaning and wiping with mineral spirits, power wire brushing, wet sanding for a few minutes, and cleaning and wiping again. It will need another one or two days of immersion to get down to shiny metal.
