Cautionary Tale: sand blasting / bead blasting on body panel

[baxsie]

As we worked on "Kick Azz", I got pretty confident with our little sand blaster running glass bead media.

The chassis turned out great:

as well as suspension parts:

We even had the suspension parts plated with Zinc Chromate, and they look awesome:

I had heard many "the heat from sand blasting will warp parts" stories, so I was very careful to be gentle on any thin parts. But I was also very skeptical that "heat" was a problem. The compressed air expanding makes things cold . . . I had a hard time believing that the tiny amount of heat generated from the impacts could withstand the huge blast of expanding cold air.

So first up, I blasted a couple of fenders and the sunroof and had them plated:

My thinking was that the zinc plating would be a great base for the 2K primer to come.

Both of these fenders had a pretty dire need of body work (I had removed a good chunk of bondo), plus the fenders have relatively tight complex curves (curved in both direction, similar to a section of a sphere, rather than a simple curve that is in one direction only, similar to a section of a cylinder) that make them quite rigid and resistant to the warping effects of the bead blasting. So my thought was that I had been "successful" with blasting and plating the fenders.

The sunroof also looked like it had been worked on and when blasting it you could see some areas had more primer+paint layers than others. So when the sunroof ended up with some "oil can" after bead blasting, I just put that down to previous work, not my blasting. If you look at the photo, you can see lots of waviness.

Then I did a hood. It looked great, or so I thought at first glance.

Then I did a door. This door had huge amounts of filler on it (1/4" plus) which I removed using a wire brush wheel on the grinder, then I proceeded to blast the entire door. That door was a mess, and then I remembered that I have three bugs, and one of the other doors must be better. So I grabbed another door and blasted it just like I had done the others. It was pretty wrinkly and I started to suspect that my careful bead blasting was causing warping.

So I started yet another door. This one appeared to have no body work, and was very straight. I carefully blasted all the tight-radius curves, checked the panel. Still straight. Using 2x to 3x my normal distance (~18", giving time for the glass to slow down before hitting the metal), I very carefully blasted just enough to remove the paint. Every few seconds I would check for warping and I found none. So at this point I have a door that is blasted all around the edges and the entire outer side, and no sign of warping.

So far, so good.

I flipped the door over and started blasting the inside surface of the outer skin. I was still using my long-distance (~18") so the blasting should be very gentle. As I just started blasting the inside of the flat panel, I watched it warp up towards me maybe 1/4" to 1/2" (this made a dent from the outside). Now having absolute evidence pounded through my thick skull that even my careful blasting was warping the panels, I shut off the compressor and tried to understand what is going on.

I then re-checked the hood. The hood is not nearly as flat as the door panels, so the warping effect was much smaller than the the door panels, but there it was: running my hand down the hood gave a wavy ride. Ruined.

I found this thread:
http://www.hotrodders.com/forum/sandblast-warping-heat-myth-76938.html

which matches my experience almost perfectly.

Except that there is no explanation why the blasting from outside did not cause warpage, only the inside blasting ?

What I think is going on is that the metal is stressed from the factory press. A normal part keeps that stress and stays springy through its entire lifetime. I remember a neighbor kid sitting on the hood of our old 1972 1302 Super, and being horrified as it buckled in, then relieved when it miraculously sprung back to its original shape.

So when the beads blast into the surface of the metal, they re-arrange it, relieving stress (anneal?).

On the outside curve of the door, the outer layer of metal is in tension. Relieving that tension does not really do anything visible, because the door is not going to suddenly spring into more of a curve. It might reduce some or all of the factory panel's ability to spring back to its original shape.

On the inside of the door, the outer layer of the skin (towards the inside of the car) is under compression. When that compressive layer is stress-relieved, it is like taking the support bricks out from the underside of a stone arch bridge. The metal collapses towards the inside, since the compressive inner layer is now relaxed. So in this one case, I think the metal actually shrunk a tiny bit, not stretched.

Can damage from a blaster be fixed?

I do not think so. That original shape that was pressed into the metal at the factory has some layers of the panel under compression, some under tension. Of course a person could pound and bondo things back to near the original shape, but the metal under the patch will never regain its factory springiness and "memory" of its original shape.

Conclusion:

1) Careful as I might think I was, if there is enough energy in your media to effectively remove the paint, then there is probably enough energy to relieve the stress of the metal, which will likely botch the part forever.

2) I think it is possible to use fine glass beads where there is a sharp compound curve (on a door, this would be everywhere except the big flat panel).

3) Blasting on one side of a panel may have drastically different success than blasting on the other side.

4) This post is based on using glass beads. Other "soft", or "light" or "fine" blast media may have completely different results.

Trying to make myself feel better:

Even though I ruined: 3 doors, one hood and a sunroof, I still had not taken to blasting the main body of the car. If I had blasted the main body I would have completely ruined the it.

After ruining the driver's doors from the 74 and 75, I discovered that the doors from the 1973 are in fantastic shape: original paint, only a tiny spot of rust. They are in much better condition and will take very little work to make them perfect.

I think that zinc chromate dipping for body panels is a good possibility, but there would need to be a way to completely remove the paint without resorting to blasting.
_________________
1975 LaGrande Super Beetle Build Log / Farm boy hinge pin puller / Farm Boy Chassis Rotisserie
"I'm not getting older, I'm getting bitter."

[CanadianBug]

If I had a nickel every time I heard someone say "the heat from a sandblaster will cause warping" I'd have a pretty little stack of nickels.
Picture this - one hundred pick hammers hammering away at an area an inch or two square. That's where the warping comes from. It's inevitable.
When the door skin warped while blasting, it's the exact same as if you had done some hammer and dolly work - striking sheetmetal with a dolly one one side and a hammer on the other will pull the metal towards the striking hammer. That's the basis of metal working.

I have to ask - why would you blast the inside of a door skin?

[skills@eurocarsplus]

[baxsie]

[MMW]

When blasting a panel it is not the heat but the impact of the beads that warps it. Each hit causes a slight stretch & with many hits it amplifies it until it warps. One way to minimize this is to hold the nozzle at an angle to the surface being blasted. This will make the impacts less severe & reduce the chance of warpage.
_________________
Mic
1959 356 coupe
Missing the original engine 72488
Update - third pc. found!
Now just need the case halves.
1965 fenderless beetle

[543fold]

[skills@eurocarsplus]

Right after blasting would have been a great time to put the epoxy down. That (in my opinion) is the best adhesion known to man.

Have you ever painted over cad? You can peel the paint off by the sheet. I honestly think you will have a epic failure like no one has ever seen.

You might get lucky, but there is no way I would ever in a million years do it that way
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[Ian]

[type241]

I want to see some PROCESS pics on the hail damage. That is very impressive work. That would have been a ragtop graft for sure.

[Ian]

[baxsie]

[baxsie]

[skills@eurocarsplus]

[baxsie]

[Der Speed Shack]

you might try sanding the ZC and then hitting it with a product called Bulldog Adhesion Promoter. it sticks to chrome, and other plating. it smells worse that a dogfart, but seems to work. you can buy a rattle can to test, if it works, buy a quart and spray i on

[skills@eurocarsplus]

i'm glad you don't see any malice, as there isn't any. i have made almost every mistake in the book, and where it comes to saving time and money, i like to share what i know.

i read thru your thread, and no matter what or how it turns out, you are spending time with your kid, and that is priceless
_________________

[Mike Fisher]

Master Series silver and/or black over the ZC would be fine for the suspension parts.
_________________
https://imgur.com/user/FisherSquareback/posts
69 FI/AT square Daily Driver
66 sunroof,67,70,71,71,71AT,72,72AT,73 Parts
two 57 oval ragtops sold
'68 Karmann Ghia sold

Society is like stew. If you don't keep it stirred up you end up with a lot of scum on the top! - Russ_Wolfe/Edward Abbey

[baxsie]

The folks over at KBS think that their RustSeal (POR-15 similar) will stick fine to Zinc Chromate:

http://techsupport.kbs-coatings.com/threads/using-...#post-1231

[baxsie]

[volksaddict]

Well the parts sure look cool with that plating on them. Just have them do the shell and the whole car will match, funky gold