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.

 

Stripping the Rear Quarters and a Lesson to Share

Steady progress has been the order of the realm this past few weeks and I am quite happy with the ground that has been covered.  With a few new parts and tools added to the inventory, I had reached a point where it was time to get the rear quarter panels stripped of all remaining original paint to see what measures would be required to get them into a clean, rust-free bare metal state that would allow proper bodywork to proceed. 

Of the options available to remove paint from exterior surfaces of the body, I prefer chemical stripping over all other methods most of the time.  Chemical paint stripper is very easy to control, does an excellent job, and does not damage or alter the surface of the parent metal at all.  By using a razor blade scraper to skim the softened paint from the metal, the job of stripping an entire panel goes surprisingly quickly even with the “wait time” involved in letting the stripping chemical do its job.

After a short ceremony to pay homage to the last vestiges of medium metallic blue color that remained on the car, I applied the first coat of stripper using proper chemical resistant gloves, respirator and eye protection.  Allowing about 20 minutes for the surface coat to completely lift from the surface, I scraped the top layer of paint easily away from the surface with the razor scraper.  Generally speaking, this would leave the primer layer(s) on the surface that would simply require a second coat of stripper to remove completely.

Once all of the original paint and primer was completely removed, I rinsed the panels clean with distilled water on a shop towel to remove any stripper residue and neutralize the stripper.  The result was nice, bare metal panels that were ready for a close inspection and evaluation in order to begin planning the repair and prep work required to plan for primer and the sequence of conventional bodywork to come. 

The panel evaluation work is best accomplished without any sort of prep coatings or etchants on the panel so that even small details are not lost or masked.  This is particularly important on 40+ year old muscle cars as many manufacturers had not yet adopted immersion priming methods or anti-corrosion primers on all surfaces of the body.  EDP primers were still decades away before the last genuine muscle car rolled off the assembly line.  A close inspection is often remarkably enlightening as was the case here.  In fact, what I realized based on my inspection was I had a very unique opportunity to educate on the hidden evils that can exist under “survivor” original paint or older restoration work.  As such, the next section will cover this critical information with some very graphic examples of “what lies beneath” in more cases than you might imagine.

Aircraft stripper is a favorite of mine to chemically strip the last bit of factory paint from the rear quarters.  One thing to notice here is how relatively "rust free" the left rear quarter appears before the stripping work begins.

The first coat of chemical stripper very quickly starts to lift the original finish and is very easy to manage as evidenced by the lack of runoff of the stripper to the side of the quarter.

Nitrile gloves and an inexpensive razor scraper are an excellent combination for removing loosened layers of paint quickly and easily with relatively little mess.  I use a grout mixing tub to catch the debris for ease of clean up and to avoid crapping up my shop floor.

A bit closer look at the large curls of paint that are removed with the razor scraper.  Also note the remaining layer(s) of primer that will require a second coat of chemical stripper to remove.  Believe it or not, there is red oxide primer under the grey coating.

The top of the rear quarter has had its first round of chemical stripping complete and you can start seeing evidence of more extensive rust being exposed.  We will detail this discovery more in the second half of this article.

Moving to the front half of the quarter, the stripping process is identical and equally efficient.  A quick few minutes with the scraper and the second coat is applied over the same area and left to work for another 20 minute soak period.

After scraping off the second coat of stripper, the front half of the quarter is clean and down to bare metal.  Again, we see more evidence of the extent of surface rust that existed under the original paint and primer.

With all of the original paint and primer removed, the left rear quarter is completely exposed for a close inspection.

Moving to the right side quarter, the same stripping operation is applied.  In this shot, both the top and front half of the quarter has been completely stripped.

Here is an interesting detail that was revealed after the original paint and primer was stripped from the right rear quarter.  In the center of the picture, you can see where some sort of light sanding was done above the wheel arch at the factory before the car was painted.  It is not obvious what sort of flaw was being repaired, but 44 years ago, somebody thought it was important.

 

What You See Ain’t Always What You Get!

Nowadays, there is a tremendous amount of hype thrown around about preserving “original paint” as much as possible and avoiding stripping a car to bare metal during a restoration.  Well………bullshit.

Quite simply, with original paint left ANYWHERE on a body during restoration, you simply don’t what you don’t know because you can’t see what can’t be seen.  As it turns out, the rear quarters on the Boss were a perfect example in the most exemplary way.  If you look at the pictures below, you will see there are hints of what most describe as “minor surface rust” starting to peek out from the very weathered original paint.

As each layer of paint and primer starts to come off the panel during the stripping process, more and more evidence of surface rust starts to become exposed.  In fact, until the last trace of the base primer layer is completely gone, the full extent of the rust propagation is hard to imagine.  In fact, a general rule of thumb is that for every spec of rust you see perforating the surface of “original” paint, the extent of rust repair requirement is roughly 10-15 times larger in scale.  In other words, if you see a ¼” rust spot on the surface, the expected repair area will be approximately 2.5 to 3.75” across and often this can be a conservative estimate.

This simple fact is shown in undeniable and explicit detail in the following photographs of the tentacles of rust spreading widely across the panel that were completely hidden beneath the original finish.  Only until the panels were stripped to bare metal was the true extent of damage revealed.  Without this revelation, there is no way a proper assessment of the requisite repairs can be completed and any hope of a successful restoration of these surfaces is simply impossible.

With that, the lesson is to strip absolutely EVERYTHING to bare metal during a restoration to make absolutely sure the hundreds of hours of labor and untold thousands of dollars of investment you will make will be spent with maximum effect and long-term return.  It really is that important.

In the next update, I will document how to deal with the “ugliness” we have discovered.  Also, I will describe a simple process to reliably detect almost microscopic rust and how to eliminate it permanently.  Also, we will start getting into the repair of some small and moderate sized dents and dings.  Until the next update!

During the paint stripping process, the true extent of the surface rust propagation began to reveal itself.  In this close-up of the top of the left rear quarter, you can just start to see how the rust has extensive "tentacles" that extend far beyond the small spec that was revealed at the top of the painted surface.

A bit further rearward on the left quarter and this "rash" of rust became more and more visible as each layer of paint and primer were removed.  If you look closely, you will see a fair amount of primer is still left in the rusted area.

This is getting pretty ugly now.  After removing only the top layer of factory paint, the surface rust has been exposed and the true extent of coverage is pretty amazing.  Very quickly, it becomes obvious that the area that was originally visible is magnified many times over once we get all of the original paint and primer off the surfaces.
 

Pure ugliness.  Careful compare the detail in this photo to the one immediately above.  The speckles of rust here and there that were visible when we started have been revealed to be an extensive matrix of rust that extended all over the surface.  Roughly 75-80% of this rust was absolutely invisible on examination of the original paint!  Had I tried to preserve this finish in any way, the results in just a few years' time would be a total devastation of a very expensive paint job and likely the loss of the original panel.
 

With the top surfaces of the quarter stripped bare, you can clearly see why it is critical to avoid getting caught up in trying to save original paint in a full restoration.  What you can't see can definitely hurt you!