E. Media Blasting Complete And The Rust Is No Longer Hidden

posted May 29, 2016, 10:38 AM by Dane Belden   [ updated Jul 7, 2017, 8:53 AM ]
It's funny how so many people who have a car that exhibits some small rust areas say "It's just surface rust - no big deal."  Well, there are cases of surface rust for sure, but most times car bodies rust from the inside out.  It is also clear that rust is usually found in places where water can be trapped for periods of time.  Both fenders on the 1970 Chevelle had rust in the lower section behind the front wheel as you can see from the pictures below.  While we don't have pictures with the paint on, many would have said it was just surface rust.


The good news is that this is very repairable because the quality of after market patch panels is quite good.  Goodmark patch panels were used (in truth, the panels are made by CHL Auto Parts in Taiwan and marketed by Goodmark, Sherman and others).  So why rust there?  Well it starts with the fact that it's behind a tire which is constantly throwing water back if driven in the rain, but the GM engineers didn't do us any favors with the design of the fender either.  Once you cut the rusted area away it becomes clear why this area is such a problem on most all cars that are 45 years old.


The fender inner structure acts as a wonderful trap for leaves, dirt and debris which tends to hold moisture for a very long time.  So with water and iron you get iron oxide which is rust.  And in this case it is surface rust, but on the inside surface - which then makes it through to present itself to the outside surface.

So the process to repair this problem is really pretty straight forward.  If doing a full patch (as done here) you want to ensure that you do two things - first you want to get all the bad metal out and the second is you want to make the cut on the CONVEX surface as opposed to the concave surface.  This is important because doing the finishing would is far easier on a convex part of the pane.  You do need to make sure the cut line is below the length of your patch panel so that you can cover your work!

Before you make your cut you will want to make a template of the contour of the fender edge that meets your door.  Now some believe it is bet to make this patch repair with the fender on the car so that you can match the door edge and rocker - that is ideal, but you can't always do that.  So in our case we made a reference mark on the fender and then made a template before we made a cut so that when we weld the new patch on we can use it as a reference.

With the bad metal removed, the next step is to cut the patch panel about 1" longer than the cut on the fender.  In addition, you need to cut the patch at the break along the wheel lip a few inches and bend the wheel lip forward so that you can slide the patch up into the fender.  The patch panel will now overlap the fender and this is what you want so that you can mark a scribe line on the fender to make the final cut for you butt weld.  Here is a shot of the butt joint after scribing and making the second cut on the fender.  NOTE: To do the scribe you need to use the wheel lip trim piece as a fixture to hold the fender and patch panel in proper orientation because once you remove the fender skin from the reinforcing structure it will move.  To ensure you have the correct amount of metal there for the fender to hold the proper shape once rewelded the scribe has to be done with the two pieces clamped to the trim contour!

Now make sure that the contours of the fender lip as well as the door/rocker edge are correct and once they are you tack, check again and then you can finish the weld.

So some grinding and a bit of filler and you are back to good.  A few other things to mention, the rust in the structure was wire brushed off and treated with phosphoric acid.  It was then primed prior to rewelding.  Once the metal finishing is done the area that was worked on will be shot with 2K epoxy primer before the filler is applied.

As can be imagined, some minor rust was also found in the door skins, a small bit in the rockers and some forward of the rear wheels in the quarters (there were silver dollar sized rust holes in the quarters behind the rear wheels which were patched with metal years ago and still very solid.)  In all of these cases the root cause was trapped debris that was allowed to stay wet and be a site for rust to begin.  In each case the affected metal was removed, areas treated with phosphoric acid, primed and new metal welded in.  

     See the debris packed in the rocker??