A common problem when prepping steel for paint is when flash rust forms on freshly stripped or blasted surfaces. This can happen because of several factors, but usually comes down to exposure to water. Wet-sanding bare metal, for instance, can cause light surface rust to form almost immediately. Working with bare metal in high humidity can also accelerate the oxidation process. These situations can be avoided by completely air-drying the work right away or by treating the metal with rust inhibitors.
For many of us, it often takes take longer than expected to return to our project after prepping metal. Life sometimes gets in the way, and it may be months before you get back to the task. Exposure to even relatively low humidity over a longer period can also allow flash rust to form.
The question then becomes not how to prevent rust from happening, but rather how to remove it again so you can finish it. There’s no single correct answer, of course, but there are several methods that are quite effective. The best answer will depend on what type of finish you need to achieve.
Below are four different solutions with tested on a piece of scrap sheet steel we had laying around the shop. It was previously used as a bead rolling demonstration panel and then put away untreated. Even though it was stored in a climate-controlled workshop, a layer of heavy flash rust eventually formed.
Phosphoric Acid
If the rust is fairly light and you want to avoid a lot of sanding or grinding on the surface, an acid etch treatment is a great solution. Here we applied a thorough coat of Eastwood Fast Etch with an acid brush to the entire area. We then used a medium (red) scuff pad to gently scrub the surface. Within minutes, the rust was gone, and the sheet metal retained its smooth surface texture.
The key is to remove the Fast Etch before it dries on the surface followed by immediately neutralizing the acid. Wearing nitrile gloves, we used paper towel to soak up the acid treatment from the metal. We then sprayed the entire surface with Eastwood PRE paint prep solvent to neutralize, wiping it off with fresh towel.
Fast Etch not only dissolves light surface rust, but it also leaves behind a protective phosphate coating to prevent future rust from starting.
Rust Converter
Another option is to apply Eastwood Rust Converter to the affected area. This product reacts with rust (but not the good steel) to permanently convert it into a paintable oxide finish. This may not always be an option for your project, as it requires significant corrosion to react properly. When it does react, however, you can watch it turn the rust from reddish brown to black.
Once the reaction has occurred, as evidenced by the black solution on the surface, it must be rinsed. As with Fast Etch, use a paper towel to absorb any excess solution. Then wipe the surface with a solvent like Eastwood PRE until all residue is removed.
At this point the metal can be primed and painted, though the surface may still show some of the rusty texture. Because of this, Rust Converter is often used in more industrial refinishing applications. Items like snowplow blades and farm equipment are good candidates, where a perfectly smooth finish isn’t absolutely essential.
Scuff Pads
Polymer scuff pads are ideal for physically removing surface contamination from bare metal. Whether used by hand or with an orbital or rotary machine, these polymer abrasives are gentler than sanding. A fine stripping drum on Eastwood’s SCT is perfectly suited to light rust removal without leaving grinding marks on the metal. Deeper rust, however, may still require an acid etch to be fully eliminated.
Sanding or Flap Discs
Abrasives like sandpaper and flap discs can physically scrub flash rust from the surface. However, they will also introduce new scratches that may require further smoothing before moving onto finishing. This solution may be necessary if the rust is more than just superficial. Before grabbing the flap disc, start with a light scuff or an acid etch treatment first.
Rust Encapsulator
A final option is to simply trap the rust in its place with a product like Rust Encapsulator. This product works by sealing off the rust entirely, preventing it from reacting with corrosion-forming oxygen and spreading. This method is a quick and easy way to treat light (or heavy) rust, especially on non-cosmetic parts. In addition to the aforementioned plow blades, Rust Encapsulator is great for axles, frames, trailers, and other such equipment.
Simplest solution is get your shop in better order to control humidity or just get busy after you are done stripping and spray some self etching primer or better yet PPG DP epoxy primer on it.
After Blast works very similarly to Fast Etch and should be treated the same way when used. It adds a surfactant to help lift and rinse away fine blasting dust. Same chemical components otherwise.
After Blast by Eastwood was recommended to me by Tommy B, one of your project advisors. Why wasn’t it mentioned in this example?
Also after reading this I’m concerned with what After Blast will do to the bare metal while waiting to be primed and painted at a shop. Will the acid damage the bare metal? I was told that after stripping the paint that all I needed to do was to apply the After Blast which will need to be removed then sealed, primed and blocked before painting.