Thursday, October 6, 2011

Preparing the roof: Paint, rust, and dent removal

In today's catch up, I'm going to remember as best I can everything I did to the roof.  This too happened over the summer.

The roof was badly damaged from numerous dents, creases, brows, and low spots all around the edges.  In fact it looked like an ogre walked along the edges of the roof with high heels.

Stripping the roof
The first part of the battle was removing paint and underlying rust.  Armed with my 4.5" angle grinder and a selection of wire wheels, paint removal is never an issue.  The primer they had beneath it, on the other hand, was and issue and was hard going to get it removed.  That and the roof is obviously a large, flat panel that involves standing, stretching, and lots of leaning with a heavy tool in your hand.

After paint removal, most of the roof was covered in black pitted rust that wasn't going to come off by any mechanical means possessed by me.  No amount of zinc sulfate or even phosphoric acid (Naval Jelly) was adequately dissolving/removing the rust.  And I was nervous about the naval jelly because phosphoric acid is not compatible with my SPI epoxy primer.  In fact it can cause very bad things to happen if not properly removed/neutralized.  I finally gave up and pulled it in to town to get the roof sand blasted though I felt it was a shoddy job and did not appear too much different.  I ended up going over it very well with another round of zinc sulfate until the entire roof was zinc gray.

The above photos illustrate the numerous difficult dents going across the back of the roof.  
If you look closely, or ENHANCE!, you'll notice the brows going along this side of the roof on the edges, and maybe even some across the front edge of the roof.

 So I introduce the arsenal, among other things:
A bullseye pick, a homemade shot bag, a flat faced soft plastic mallet, a slapper, and a quality semi flexible Durablock sanding block were among the most commonly used tools to fix the roof.

Dent Removal

Dent removal, if you're an amateur, is just something you're going to have live with as something not pleasant, especially on a roof.  The metal roof is a large flexible panel, supported by a measely support going across the center on the underside (which needs to be removed by the way to get access to all your dents), and the natural crown, or curvature, of the roof.  Merely pounding on the roof just makes the metal flop and bounce.  To restore the integrity and structure of the roof, the crown of the roof needs to be restored.
And it don't start very pretty.  All those giant dents in the back were first attacked with several very sharp blows from the underside with a very sharp pick hammer.  This created high spots in the metal, but it also restored the crown and made them metal easier to work with.  The high spots can be shrunk out or tapped back down with the slapper.  More on this later.

The next point of attack was remove the brows along the side edges of the roof and along the front.  I foolishly began the left side by shrinking down the brows with a shrinking disc.  This resulted in incredibly low spots that I had a very difficult time getting out (and the metal never was the same, by the way).  Instead, on the right side, I merely tapped the brows down with my slapper (the best body tool I have) and it was perfect.  Like nothing had ever happened.  Low spots were targeted and carefully worked out with the bullseye pick, with tapping from a dolly underneath, or even by pressing up on the low spot with one of my pry bars.  There are numerous ways to skin a cat and you'll think of them all when trying creative ways to pop up low spots, especially when you lack the arsenal of tools possessed by a pro.

So going back to the rear edge of the roof.  The worst of the dents was popped up.  Now there existed numerous low spots to fix.  Since there is a ledge on the underside of the roof that doesn't allow access for a hammer (and it would be utterly difficult to remove), this was best remedied by sliding a long pry bar in between the ledge and the roof skin and prying up on the low spot.  Now from the top side, tap on the metal around the low spot.  This pulls the low spot up and pushes the high spots around the low spot down, reshaping the metal back to normal.

So let's see how it came out:

The brows on this side came completely out.  Only minor filling will be required on this side initially.

In this picture, the back of this roof now looks as good as new, especially when compared to the originals posted above.  But you can't see what I can still feel when running my flat hand over the panel, so there will still be extensive filling required here.

The next steps were to add filler to fill in low spots that I can't work out and smooth out the panel.  This filler is your basic 3M lightweight filler applied to bare metal, then sanded 40-80-180/220.

Filler work along the front

The minor filler work required on the passenger's side
There gets to be a point where I know there are still low spots but they're just too hard to visualize, so I finally apply the first two coats of epoxy to the roof to see how I did.  Upon first glance...excerrent!
 At this point the epoxy was guide coated (I use a cheap black spray can primer) and blocked with 220 to find the low spots.  A lot of people cry about the sandability of epoxy.  SPI epoxy sands great.  To my dismay, my roof look like a dairy cow.  Numerous quarter sized black spots were present.  These were worked out with the bullseye pick until the black sanded away with blocking.  Larger, wider spots required the use of a shot bag and some persuasion from below.

I constructed my own shot bag by walking down the hill to the river and grabbing a few handfuls of sand from the bank, then baked it in the hot sun to completely dry it out.  I stuff this into a microphone bag (readily available if you're a sound engineer as I am), duct taped up the edges/seams, and I had a pretty nice little shot bag.  I used the shot bag as a forming dolly on top of the roof and gave persuasive blows from the bottom with my flat faced plastic mallet.  I also worked these spots additionally with the bullseye pick in a spiral manner.  I didn't want to go too crazy though.  If I could tell I wasn't making progress on the spot after trying my best, I cut my losses and would leave it to be filled.  I'd rather do that than make a worse problem for myself.
This picture was actually taken a little later in the process than what I was just describing but you can still see some of the little black spots on the right side to be worked out.

I then added the necessary filler to what low spots I found and couldn't work out and shot the roof with another 2 coats of epoxy.
The final product before I applied another two coats of epoxy.

A few days later I shot the roof with the first three coats of SPI 2k building primer.  This was guide coated and blocked down with 220.  Small low spots were bumped out.  At this point I went for broke since there was some funky stuff going on in the middle of the roof and I skim coated a good portion of the roof.  Since the filler was now going on 2k primer and not epoxy, I moved to a polyester filler, which we will call Marson Platinum.  This is analagous to Rage Xtreme but half the price.  It's a very smooth, creamy filler that's exceptionally easy sanding and is really great for skim coating.  This only needs to be sanded down with 180 or 220.  The edges were feathered very well and I worked the edges out with Evercoat EZ sand putty to blend it into the rest of the car better.

Skim coated roof.
The skim coated roof now gave me plenty of material to work with to sand down until I could barely start to see metal.  At this point I had pretty much eliminated all the ripples.  Once again, the roof was shot with another 3 coats of 2k primer and blocked as before.  A few remaining low spots from imperfections in the skim coat were filled, and putty was applied to deep scratches.

Finally, the [hopefully] last three coats of 2k were shot but never sanded, and that's just how the car sits today.  Hopefully, when I can get back to working on the car someday, it should be almost perfect upon blocking and ready for the next step of wet sanding.  I guess we'll see.


  1. Amazing work! Good job! Thanks for the update on your blog.

  2. Amazing work, indeed! How is the restoration project going? Are you still working on the mustang, or is it done by now? I can’t wait to see it fully restored and running. Good luck!

    Joel Matheny @ Indiana Dent

  3. Thanks Joel. Nope, still not done, but getting there. Lots of updates I've posted since this.