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SouthernMudSlinger
07-20-2006, 04:54 PM
I've started doing body work on a fender for our car (1970 plymouth) and i got it blocked down.............but, there is a bunch of high places. We sent all the parts on the car to a place that does rust repair and they media blasted it all and fixed the rust on the pieces that had rust. They explained that you dont want to media blast both sides of sheet metal, that it will mess with the tension of the metal. They blasted both sides of this fender. I've got a couple pics.................it looks like the media blasting created the high spots, cause they are all about the same size round, and alot of the high places run around the fender. What you see in the pics are the tops of them that are just exposed (they are level with the top of the filler). The filler is only about a 1/16" thick. When we stripped the filler off the car, (before we took them to get rust repaired) there wasn't hardly any filler on the front fenders, just a couple spots, and no evidence of previous body work (no grinder scratches, that i can remember. Up towards the front of the fender, i've started tapping the highs down and sanding the filler off as the highs are tapped down.
http://i73.photobucket.com/albums/i232/Southernmudslinger/b960d2d7.jpg
http://i73.photobucket.com/albums/i232/Southernmudslinger/3845c6c7.jpg
http://i73.photobucket.com/albums/i232/Southernmudslinger/d4e65eff.jpg

Looks like i got a mess to fix. What do ya'll think? was the high places caused by media blasting (from looking at the pics)? Im not sure what media they used, but in their booth, there was a bunch of some sort of brown media that they used.

MO-Cummins
07-20-2006, 06:19 PM
Well soduim is white. So your sure it wasnt some sort of sand? What did the panel feel like when you got it back was it real smooth or real gritty? Sodium blasting leaves a panel pretty smooth, compared to what sand would do. What usually causes this warpedge is heat that builds up do to the blasting + the air pressure.

P.S If it was Soda i would wipe the panel down with vinager or equivilant or nothing will stick.

Lost in NJ
07-21-2006, 07:50 AM
It does not look too bad to me. If you have areas that are pushed down slightly they will have circular shapes. You have a panel that came from a car that was used for years so I would expect anything. Plus you have high spots along the edges where they are less likely to cause damage from pressure blasting. In my none expert opinion I do not think it is from the blasting, it looks more like wear and tear. Either way, it does not look too bad.

Have you mounted this panel on a car with the doors to be sure all the lines line up? That is a very important step to prevent surprises later. Nothing worse than leveling a panel only to find you need to bend it some and re-level. Been there done that.

BTW, it is a false concept that heat causes warpage when blasting. It is the physical hammering effect on the surface of the metal that causes warpage. To shink metal by heat you need to get it close to 500* F or so and the expanding air stream from the compressor is very cool. But if you hit the surface of the metal with lots of little hammers you can actually expand just that surface of the metal causing all kinds of grief.

You could fix the metal if you do not want to level the panel with bondo. Using a long board or a flat file you can locate the high and low spots and work the metal flat. It is kind of time consuming to do as you learn, but it is quite satisfying to do. You will need to make or buy a slapper and consider getting a shrinking disk.

MO-Cummins
07-21-2006, 01:00 PM
BTW, it is a false concept that heat causes warpage when blasting

One thing i did do alot of is blasting. Not necessarily when you soda blast but when sand blasting the next time you should sand blast a spot and then take time to feel the spot right after your done. I have seen heat build up from a line sander cause warpedge, but who am I.

Lost in NJ
07-21-2006, 01:21 PM
I think I should add I have sandblasted 4 whole cars and never seen warpage. Based on what I have read from others the cars should all be pretzels by now.

A belt sander could produce the temps to do localized shrinkage. I suggest you read about using sanding disks to do shrinking like is done with a shrinking disk.

To shrink metal by heat you need to get roughly to the point where the metal starts to turn blue. At that point the metal is plastic enough that the cooler surrounding metal can push the metal into itself. When cooling the metal drops below the plastic point before the pressure is released so the shrink condition stays. This is also the reason why every weld is a shrink point.

MO-Cummins
07-21-2006, 05:36 PM
I refuse to continue to debate anything with you. I have blasted only about a dozen full vehicles and atleast twice that in just panels with silica sand and sodium, not to mention boats parts ect.. I can assure you i have seen plenty of warpedge i congradulate you on not having any warpedge must be nice i however wasnt so lucky on my side of it. I see these sites as a way to try to help people learn stuff they do not know not a place to argue over something that really means nothing to me. I have heard a ton of blasters say that heat causes it+ i have had warpedge with blasting, so with that in mind i just put 2 and 2 together to come up with my information. If yours is some kind of proven fact, so be it. I guess a person learns something new every day.

Lost in NJ
07-21-2006, 06:50 PM
There is no debate here. I am trying to offer advice based on experience and knowledge that can be verified by science. Not word of mouth.

I hope you took the time to understand what John Kelly replied on the autobody101 forum as the advice you gave makes the problem worse. It is kind of neat to tig in a panel and planish the metal so only need a very thin layer to level. That is what I try to do, but it has been a long road to get to this point.


http://www.autobody101.com/forums/about5885.html

Kevin

My Project (http://http://home.comcast.net/~68c/)

MO-Cummins
07-21-2006, 11:29 PM
I feel that everyone can always learn something. However in that instance on a complete different site my bad judgement in advice was due to my hastily reading and not taking the time to understand fully the question. Again that was my fault. That is why i did not reply to argue his techniques. Once i fully determind the situation i could plainly see where i went wrong. In that specific post, after a days of work and being tired i just seen the word "DENT" and that was about all i comprehended. As with a normal dent you see my advice is just, fine that i know cause i have done it alot of times. As matter of fact i learned it from someone that has been doing this sorta thing for many many years.

Anyway i apoligize for all the misconceptions.
Matt

Phil V
07-22-2006, 01:22 AM
I agree with Mo cummins. Lost in NJ - With all due respect your flat wrong on this one. In most cases using a bodyshop type pressure feed sandblaster on sheetmetal like a door or a fender WILL heat warp the metal. If you didn't heat warp metal on cars you sandblasted then it was pure luck on your part.

Lost in NJ
07-22-2006, 04:19 PM
Read this post from another board:


http://www.streetrodstuff.com/Roundtable/Archives/Year_2001/October/88.php

MO-Cummins
07-22-2006, 05:53 PM
Its a guage conversation. You sure thats the right one if so ill read it.

Lost in NJ
07-22-2006, 06:41 PM
OPPPs

I am sorry the copy paste did not work right.


This link is the proper one.
http://www.hotrodders.com/forum/sandblast-warping-heat-myth-76938.html

Phil V
07-22-2006, 08:28 PM
Just for the sake of discussion - if you take a bunch of little hammers and hammer a piece of thin sheet metal - Does the area you're hitting with the hammers dent inward or protrude outward ? Most reasonable people will agree you dent the metal inward. Having said that, now take an oxy/acteylene torch with a # 1 tip and heat a spot red hot the size of pencil. does that spot create a dent inward or does heat warped metal protrude outward ? While keeping in mind that just about all warped metal from a sandblaster protrude outward. Have you guys ever watched metal being sandblasted in a low light condition ? There are white hot sparks as the sand bounces off the sheet metal.

Len
07-22-2006, 09:29 PM
I agree that it's not the heat but it's the thousand little hammers hitting the surface that causes the warpage but no matter what it is you can bet that it's easy to warp a panel. I've seen cars destroyed by an enthusiastic owner with little experience trying to strip a car using a small blaster. This is one of the reasons I recommend using a good variable speed sander/buffer to remove the paint over most of the car then you can use the blaster to get into the hard to reach places.

MO-Cummins
07-23-2006, 11:26 AM
OPPPs

I am sorry the copy paste did not work right.


This link is the proper one.
http://www.hotrodders.com/forum/sandblast-warping-heat-myth-76938.html

That was a intereresting write up. As i said that theory may be 100% true i just went of what i was told combined with what i seen. If i am wrong i am wrong not ashamed to admit it.

At anyrate, Len makes a great point. When sandblasting it is very easy to warp a panel for whatever reason. Due to the cost, and the work that MAY be invovled once you recieve the panel back, for a novice or maybe just plain in genral it would be better to go another route. Unless the specific blaster is very good and never has a warpedge problem. I know in my state it is not a good idea in the legal eyes anyway, to blast with sylica sand anyhow. Sylica has serous health risk problems.

Anyway this was a good informational post in my eyes.

Lost in NJ
07-23-2006, 08:29 PM
Philv

There are some good reasons for what you observe.

When you hit with a hammer the energy of the hammer is great and there is more than enough force to move the full thickness of the panel.

The thousands of pieces of sand are not massive enough to move the full thickness of metal, but can affect a small distance into the metal. This causes the metal to expand on the side it is being blasted. This causes the warp to come towards you.

With a torch the reason the warp comes towards you is because the metal has a tempurature through its thickness as it warms up. This means the side that is getting the heat will expand slightly faster than the other side. This causes the metal to start the warp towards the flame. As the heat becomes more uniform through the thickness the warpage has already been started in the one directions so it continues.

The sparks you see are the same you get when you throw a rock against another rock or the railroad tracks. I believe that spark is the result of the energy being released in breaking apart the crystals. There is a lot of point heat generated in fracturing the sand. Little of that energy makes it into the panel.

G_T
07-23-2006, 10:46 PM
Nice Superbird fender! Keep the sandblaster away from the rest of the flat panels of your bird... please!

CoolasIce
07-24-2006, 03:44 PM
How can you tell it is a Superbird's fender. I was thinking it's a Satellite's fender.

G_T
07-24-2006, 09:43 PM
Two ways. It has holes in the top for mounting the fender mounted scoop. Second, the fender has a large cut for the valance.

CoolasIce
07-25-2006, 08:54 PM
GT, I see the holes you are talking about.