Removing Surface Rust & a Tip on Exposing and Eliminating Micro-Rust

When we left off last week, I had managed to get both rear quarter panels stripped to bare metal and I shared some detail on the extent of the surface rust that was hidden under the original paint that was exposed during stripping.

In this update, I have set out to remove most of the lighter surface rust with my angle grinder fitted with a surface conditioning disc.  These open-weave nylon abrasive discs make quick work or removing light rust and surface imperfections without damaging the parent metal.  Fortunately, the vast majority of the surface rust was easily removed in this way, but it also revealed a few concerns where the heavier rust deposits were.

Close inspection after cleaning up the visible rust revealed an area of tiny pits (and a few slightly larger ones) that appeared to have cleaned up.  However, experience has taught me that surface conditioning discs do not clean rust from pits very well and in all likelihood, there was plenty of microscopic rust (a.k.a. micro-rust) still living at the bottom of these tiny pits.  The problem is if you can’t see the rust, how do you eliminate it?

To many, the quick and easy answer is to haul out the sand blasting rig and assault the panel with high-velocity abrasive to remove this miniscule rust residue at the expense of all of the good metal around the areas.  Unfortunately, this is analogous to driving finishing nails with a sledge hammer.  It’s kinda the right idea, but the execution is never what you really had in mind.  However, the most effective solution to address this problem is actually simpler and cheaper than you might think.  Stay with me now……

As I have outlined many times in previous blog entries, I use a phosphoric acid Prep & Etch solution to treat and preserve all of the bare metal that is exposed while I work.  This serves many useful purposes, not the least of which is keeping flash rust in check and arresting the progression of existing rust by converting it to black ferric phosphate (FePO₄) until it can be properly dealt with.  The problem here is that once the exposed surface of the rust is converted, the conversion reaction stops and the underlying rust, while essentially arrested and sealed, is still there.

But hold on a minute and let’s think about this.  If we have taken our time to prepare a nice, clean, bare steel panel by removing all traces of original paint, brushing the visible rust off the surface with a surface conditioning abrasive, sanding the surfaces smooth, and finally applying a light coat of phosphoric acid prep to preserve the panel, what more can be done to positively identify micro-rust and how do we eliminate it effectively without tearing up a perfectly viable panel?

The answer is almost ridiculously simple.  Remember how the phosphoric acid converts the rust it contacts into a black inert byproduct (ferric phosphate)?  Well why not use this very reactive characteristic as a chemical “indicator” of every spot on the clean panel that retains rust?  In other words, the black byproduct has now become a very effective and relatively high-contrast “developer” that will highlight every spot where micro-rust remains.  So, by using a phosphoric acid prep solution on your clean steel panel, you will have created a virtual “road map” of sorts that clearly shows you all of the areas that will require addition cleaning methods to remove the remaining rust.

Now that the “secret” of exposing the remaining micro-rust areas has been identified, the process of removal can begin.  This is where the process becomes a bit of a throwback to the popular sand blasting method but on a much smaller scale.  Since blasting is so effective at removing rust and other surface coatings, I have come to love a product from Zendex called the Speed Blaster along with their add-on accessory the Hot Spot abrasive recovery fixture.  Essentially, this combination creates a small, virtually dust-free miniature sand blasting apparatus that will blast small, dime-sized circles of metal with high precision and effectiveness and it only requires a few minutes to master.  By using this tool to blast each and every black “freckle” indicated by the phosphoric acid solution, I can very methodically remove even the tiniest traces of rust from pits that are almost invisible to the naked eye.  And the beautiful part is the method of checking progress is to simply reapply the phosphoric acid solution to the freshly blasted area and look to see if the black spots reappear.  If any black traces remain, I simply fire the Speed Blaster back up and repeat the process until all traces of micro-rust have been eliminated.

Once the job is complete, I treat the entire panel one final time with a phosphoric acid wash applied by hand, allowed to sit for a few minutes to etch the metal surface evenly, and then wipe off the residue with clean cloths and quickly hit the entire surface with compressed air to drive off the moisture as quickly as possible.  At this point, the panel is preserved and can be worked extensively in preparation for dent removal, fabrication work, metal finishing, and structural and cosmetic filler work.  Yep, that’s where we are heading next!

Using a surface conditioning disc on my angle grinder, I was able to remove most of the surface rust spots that were exposed once the quarter panel was completely stripped to bare metal.

I knew the surface conditioning discs would not remove the micro-rust in the tiny pits that were revealed where the heavier rust was.  I use phosphoric Prep & Etch as an indicator of where this nearly invisible rust remained so I could remove it.
To keep the phosphoric acid from getting everywhere, I use a folded paper towel to keep the acid localized in the area I am trying to treat.

Looking closely, you will see the phosphoric acid has turned the micro-rust black and it shows up quite easily against the shiny bare metal.

Now that the micro-rust can be easily seen, I can confidently remove it.  a bonus is that the phosphoric acid will continue to perform its "indicator" role for as long as necessary until I am able to remove ALL of the hidden rust.

A quick run over the entire panel with an 80 grit disc in the DA sander polishes up the metal surface nicely and adds contrast to the areas that we have treated with phosphoric.

This is a dandy tool called a Speed Blaster that allows small-scale sand blasting in your shot without making a mess.  With the proper rubber nozzle on the Hot Spot recovery unit, you can sand blast dime-sized spots wherever they are needed.  This is how I tackle getting the hidden micro-rust off of the quarter.

Here is a close-up of the spots blasted with the Speed Blaster tool.

Here is a shot of the quarter after I have blasted all of the spots that showed the black ferric phosphate coating left on the surface of the hidden rust by the phosphoric acid.

After wiping the entire panel off with acetone, I treated the surface with phosphoric acid again to see what spots I might have missed.  Fortunately, there were only a few small spots that required a bit more blasting to completely remove the micro-rust that remained.

A final treatment with phosphoric acid Prep & Etch and the panel was ready for formal bodywork without the worry of having missed any hidden rust.