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I'm looking for any good advice on how to minimize warpage caused by media blasting. I picked up my long term AE86 project from the blasters last friday and was confronted with a warped roof and trunk lid. Knowing a few other AE86 projects that have had this happen to them I thought I could avoid it by manually stripping most of the paint off in advance with one of those spongelike stripping discs. Sadly I did not go far enough and they ended up putting too much heat in several of the bigger flat panels (Toyota used a very thick and resilient paint system it appears, good resistance to blasting, not so much against rust) Anyway, I may be curse word off at them for getting it wrong and myself for not doing all I could to minimize the risk but it wont change the situation the car is in so I prefer
to just prepare myself to fix this or have it fixed (since I do NOT want 5kg's of bondo on the roof).
Initially I thought that the problem I'm combatting is oil-canning, but I'm thinking it would be more logical for the heat to have caused localized shrinkage of the metal, which fits with all the warpage being depressions. Here are some pictures of the roof (all I have at the moment): Some final remarks: all dents will more or less spring back into the proper shape when I push them from the inside, they just wont stay there. Any suggestions for methods of remedying this type of damage are most welcome!
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Get a can of freeze spray.
Warm up the dents with a heatgun and push out to the shape you want, then while holding the metal in position use the spray to cool the area around the dent, and then the dent itself.
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1988 Mercedes w124 superturbo diesel 508hp 1996 Mercedes s124 e300 diesel wagon 1990 BMW E30 V8 M60 powered! 1999 BMW E46 323ci project car
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Thanks, that sounds like something that's easy to try and couldn't cause any new damage so it's going on the list.
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This is going to be difficult to achieve regardless of what method you choose - for one I would not recommend heating then cooling rapidly has this is likely to lead to further distortion rather than reducing it - it will also anneal the metal which could lead to metal stress problems at a later date - very careful shrinking with localised heat then light use of a hammer & dolly can reduce the effect although over such a large panel you are not going reduce it to nothing - you will always have to be mindful that some use of a polyester filler shall be required - YouTube link below to a old school craftsman which is how I was taught - but you really need to know what you are doing and have a bag full of confidence to undertake this effectively You may well be better off seeking proper professional advice on this before you proceed further - finally a link to what I had to do in order to correct someone's previous repair on a roof of a current restoration that is in my workshop forum.retro-rides.org/thread/200957/1967-corsair-restoration-enroute-painting?page=19Post up any further questions and I shall do my best to answer them for you - but I would be weary of attempting repairs of this nature if you don't have the experience of undertaking this type of repair - it is all too easy to make the situation much worse and I doubt that roof skins for the model are readily obtainable
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The roof skin was a replacement I welded in, I doubt I'll find another anytime soon. I don't mind filler, I just want to avoid having to apply thick slabs of it. Finding a pro is not an easy task (am in the Netherlands). I would prefer to get some practice in before touching my roof but it's not easy to find a practice piece.
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Look up using shrinking discs, they are supposed to be easier to use than a flame but still need a fair bit of skill and it would be easy to make things much worse.
Its not the heat from the blasting which has caused this, blasting puts a compressive force into the surface of the steel effectively stretching the top surface causing it to ripple (same principle which strengthens gears with shot blasting).
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I have a large and small model shrinking disc at my disposal (but I lack the humongous grinder required for the large disc). Am already reticent about using fire to shrink (lack of experience).
My current thinking is this: Get dents convex by pushing up from bottom and lightly tapping around the rim of the dent with a bodywork hammer.. see if I can get the dents to settle in the convex position. From there, if there is a noticeable bulge try to shrink it down from the center with the shrinking disc. I will be trying this tactic out on the trunklids first. And again, it doesnt need to be a metal-finished result, just serviceable with non-ridiculous amount of filler and more importantly no oil-canning that might fire said filler into the stratosphere on a warm day.
Think you might be right with the stretching by peening point of view as main cause of the warping.. Yes blasting might add heat but the amount of air delivering the blasting medium more than likely cools the metal as well.
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Apr 10, 2019 11:15:15 GMT
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I had a similar issue on the bonnet of our minor, given i had nothing to loose I tried heating hammering towards the centre and quenching in the centre of the oil can, the result was far from perfect but it got rid of the oil can and got it down to a sensible amount of filling.
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Apr 10, 2019 14:28:07 GMT
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I fell your pain. In my case, the car was a write-off because the blaster had done so much damage. Not just the roof panel, but pretty much every panel on the car. But cheer up cause it went from this To this: ( albeit with a skim of filler over the entire panel) The first thing you need to realize is that heat has nothing to do with sandblasting damage. I actually measured temp while deliberately destroying a test panel, no temp change. I did a LOT of research when trying to figure how to solve this, and what happens is that the high energy sand particles deform the top layer of the metal, causing the surface area to change. I cant tell you for sure if it gets larger or smaller, and it could be that both happen at random depending on gun angle. But changing the surface area of one side of a panel and not the other, means that something has to move and that is where the warp comes from. Fixing it is a curse word. You need to use a shrinking disc and pray that you don't make it worse by shrinking in the wrong spot or shrink too much. I have a large grinder now, but did a lot of this kind of work in the past with a 4.5" grinder and a large wheel...scary as hell but it works. This video on youtube was a huge help as he shows how a bit of pressure outside of the oilcan will show where to shrink for best odds of a repair. This technique worked for me on many of the oilcans on this car. In some cases you might over-shrink and make things worse and the only action is to use a large broad slapper made from an old leaf spring and put CROWN back into the panel. Took 400 hours of disc shrinking, hammer/dolly beating and sanding to get it to this state. Good luck.
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Last Edit: Apr 10, 2019 14:30:21 GMT by bjornagn
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Apr 18, 2019 12:35:24 GMT
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Been reading your project thread last week.. that molvo is a piece of art. Found the following useful discussion on the subject of blasting damage: www.jalopyjournal.com/forum/threads/sheet-metal-sandblast-damage-a-test-subject.658008/With pictures someone made of a trunklid that was intentionally blasted to warped state and then repaired using different methods..(heat, shrinking disc, donut dolly). The guy also did infrared temperature measurements during blasting and confirmed the metal does not in fact get hot. Looks like I will have time for a first go at my own trunklids this weekend.
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Apr 18, 2019 18:31:44 GMT
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Ivan:
Thank you for the compliment. Been quite the adventure building the Molvo and I have learned a lot along the way. Mostly that I might be a bit crazy!
The jalopyjournal article that you mention was instrumental in my understanding of the nature of sand-blasting induced warp; its the reason I actually did my own temp check while blasting. I can't say however that it was all that directly helpful when it came to fixing the many oilcans on my car. The other thing that the article taught me is to not take any internet advice at face value until you are clear in your mind WHY it would work.
For the most part, each oilcan seemed to have it's own personality, sometimes two oilcans would be related in that pushing one moved the other. The method of putting the oilcan in the correct side and feeling around for the "lock" was a major breakthrough. Looking at your issues, I would start that way.
I screwed up with the disc more than once where I either shrunk in the wrong place or shrunk too much. Nothing to do then but slap some shape back into the panel and do whatever forming is required.
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Apr 18, 2019 19:23:06 GMT
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I wonder what would happen if you had to aggressively blast the inside of the roof? Wonder if the stresses would equal things out and pop the panel back in shape?My gut feeling is the blasting stretches the material,leading to oil canning.
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Last Edit: Apr 18, 2019 19:25:07 GMT by Deleted
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Apr 18, 2019 19:59:00 GMT
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I wonder what would happen if you had to aggressively blast the inside of the roof? Wonder if the stresses would equal things out and pop the panel back in shape?My gut feeling is the blasting stretches the material,leading to oil canning. That is actually a valid remedy and a friend of mine who reconditions farm implements for a living says he uses it all the time. Suggested I might want to give it a try. I passed! My thoughts landed on the possibility that the metal could be stretched OR compressed due to the blasting. Think of a layer of clay and how it would move and bunch up in some places but thin in others. At one point, I peeled the door skin off and ran it through the E-wheel thinking that would be the easy button. Oddly enough it did not solve the issues since even with just a wash of pressure on the panel it wanted to rise too much. This suggest that the overall effect was a stretch. I don't think you can ever get to be perfect at fixing these things, just develop the confidence that if one thing does not work the next will and refuse to quit.
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Apr 19, 2019 18:16:14 GMT
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Well, Went out to test drive a new daily near the lockup so I swang by to give the newly bought panel hammers a try. Here's the absolute worst part in terms of visible damage that came back from blasting, my spare trunklid: As you can see, it's all bulging heavily inward between the support structure so I'm assuming this happened when they blasted from the inside. The trick with blasting both sides of a flat panel to get it back to flat again is something I have heard from both the guys that did this and the bloke that did the blasting and powdercoating of my suspension parts. The trouble is the sheet metal is not free to move, and when you stretch the metal from both sides, you'll still end up with a stretched piece of metal. Anyhow, they tried that and it obviously did not work out. Keep in mind I have never done any panel beating before so I'm assaulting this thing with the mindset that it's a ruined part to start with. Here's the status after 2 hours of beating it with a panel hammer (like a hyperactive toddler): Different angle because the light was not so good anymore in the previous spot. You may notice it still looks like curse word but it now looks more like a uneven field rather than a skate park. Only thing I've done thusfar is hit it from beneath around the edges, repeating until it comes near to flat (should have some CROWN I suppose). No dolly since I've no where to put it (the support structure is my dolly). I also hit the ridges from above, but gently to avoid stretching, it does seem to help a little in some cases. I had to head home again so I did not yet get started with cleaning up and shrinking with the disc. There is already an oilcan or 3 in there now that could be addressed. I'd like to get all the dents raised before I start that though..
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Apr 19, 2019 18:26:40 GMT
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Looks to me like they did it from the outside and it pushed in where it does not have any support frame on the inside .
Are they not supposed to do it at a shallow angle rather than 90 degrees to the panel ? That and more likely they have the pressure too high so they can get a job done quicker , as well as maybe a too aggressive media . Should be baking soda or something simialr.
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Apr 19, 2019 19:57:37 GMT
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That is pretty much every panel on the Molvo project. You are approaching this with the right mentality. No hardfast rules and you wont get perfect,but good enough to skim.
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Apr 19, 2019 19:57:59 GMT
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Its very rare that I will send a panel for blasting now days - it just makes no end of additional work - the vast majority of my panel work goes for dipping and is returned undamaged - more to the point a dipping process removes all paint & rust from any cavities - something that blasting cannot achieve
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Apr 19, 2019 20:22:08 GMT
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Should have stripped the sensitive panels myself, but, as we Dutch say, it's easy to look a cow up its backside.
Bought a construction stamp to help with the roof (one of those things to hold beams up) and a long ruler to help find high and low spots.. Plop the dents up and massage away. But I'm not touching the roof until I've found out what works for me with the trunk lids and the one door they finished. (was already warped from welding, the other better one they returned, afraid of damaging it).
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Apr 19, 2019 21:21:38 GMT
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In relation to the bootlid would you not consider getting a secondhand one? Save the hassle of all the panel work/straightening
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I might, but they all have rust in the seam. The point of the boot lids is to get practice in for the roof.
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