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MG MGA - Fender challenge

This is my RH front fender. It was dinged and repaired by PO by lots of bondo, at least 1/2". It looks Ok from the outside, but I am tempted to cut it out and weld in a new piece. I hate to leave that crumpled bit. This area seems to be a flat curve, not compound, and I know it will need a bit of filler after I'm done. What does everyone think? Any tips? I was thinking of cutting out, leaving about 1" at the edge where the bulkhead slips in, and hammering this to the right curve (template it beforehand)and re-fabricating the bulkhead, welding it to the remains of the existing one. I don't want to have to do the folding at the edge again.

Art Pearse

some hammers and dollies... will get most of that out ..

k klay

Art, I think a skilled panel beater would be able to remove the dent without resorting to cutting. The area is clearly severely stretched and a lot of shrinking will be needed to get it back.
If you just resorted to hammering you might end up with the dreaded 'oil can' effect.
If it was mine I think I'd have a go at shrinking the dented area using a shrinking disc. First you would have to remove all the filler on the front side and then apply the disc from the back side. You would be surprised at how much you can achieve using this method. I made myself an 8" disc and I've just bought a 5" one from the US. If you can borrow one or buy one cheaply for a trial I would recommend it. Have a look on Youtube for examples of shrinking disc use.
A professional would probably use an O-A torch for shrinking but not many of us have the equipment to hand..............Mike
m.j. moore

Art, if you're game I would try to beat that out and use a shrinking disc to remove the stretched metal once the panel is close to flat. You can get a nice small shrinking disc for a 4.5" grinder on line for around $40 and there are videos on how it's used. I have one and it's pretty easy to use for an amature metal beater. Even if you screw up you're only out the price of the tool, you can always cut it out or take it to a good panel man. I wouldn't bother with most body shops these days though, no one straightenes panels anymore they just replace them, it's almost a lost art. http://www.eastwood.com/eastwood-shrinking-disc-operataing-instructions
B Young

Art-
Of you cut the piece out, you will at best still have nearly as much distortion as in straightening, and likely more, plus the problems of having a weld there. I'm assuming you are MIG welding, which is not nice for subsequent hammer work. I'd straighten it. You will need to get the bulkhead out of the way in either case.

Tips:
Work opposite to the dent creation, ie from outside in.
Any time the hammer hits the dolly - CLINK - you are stretching metal.
If you hammer on a high point and it goes - thunk - you are shrinking metal.
High points can be shrunk with a propane torch. Heat the high point red hot, it will raise up. Hammer flat, takes one blow with the hammer and dolly. As it cools the now partly shrunk piece will shrink more. Do a couple of points then cool the area with wet rags. This will also anneal the work hardened metal where the worst of the dent is. If there is a long scrape sort of dent, work along the length of it. Don't over do it, but a bit of excessive shrinkage here will give you some slack for subsequent planishing - clink, clink. Planishing will re-work harden it, but don't overdo and re-stretch it!

This can be done to the point where you need nothing but a good coat of high build primer to finish.

FRM
FR Millmore

I am encouraged to try beating it out! Couple of questions - FRM, you say beat it out opposite to the dent creation. It was a hit from outside, so I should beat it from inside, dolly outside? But you say outside in. Also, I need to remove the bulkhead, straighten it or remake it, in a way I can put it back again. It seems to be held by crimping at the trailing edge of the fender, plus a couple of spot welds at the top. Can I prise up the seam to remove it? With the bulk head out, I need to make a template of the curve first, maybe from the other side fender which was not damaged.
Art Pearse

Art-
Well yes, you hammer from inside when possible because it is much easier, but my reference is to the sequence of the event. You try to reverse the bending of the metal in time - as in a reversed movie. The major reason things are difficult to straighten is that the last guy started by smacking the dent in the center, giving you not a straighter piece, but a new dent in the opposite direction superimposed on the old one. The new dent then distorts the old one, "locking" it in place. I have had the fun of hammering the last guy's attempts back into the original damage shape, then straightening that!

If you push on a panel, it starts to bend at the pressure point, then the deformation travels outward as the dent gets bigger. So you start straightening from the outer circumference, and spiral inward. This part of the work gets done with a rubber mallet (or fist) with a board for backup if there are no sharp locking ridges. Ridges and shrinking call for hammer and dolly. Always remember that CLINK means you are both stretching and work hardening the metal. Gently, gently is the word of the day. Much better to think of "ironing" or "massaging" it out, rather than "beating".

If there has also been local stretching, like when a bumper corner gets into your door, the main big dent happens first, and the stretching damage only happens after the big dent can support the metal in the center enough to stretch it. So, you shrink the stretch damage, then work the big dent as previous. In bad cases, you need to alternate back and forth between the two modes. In a directional scrape/stretch (what you seem to have), you would start shrinking at the terminus of the scrape, work back along the damage until the scrape is gone, then work the remaining "smooth" dent out from outside in.

Bulkhead:
You drill out the spotwelds, and then gently tap the bulkhead out of the crimped edge. You want to minimize distortion of the edge until you get things straight. If you try to pry it up, you are adding "locking distortion" to the original damage. With any luck, the bulkhead will straighten out to exactly the shape it is supposed to be, which will automatically give the correct curve to the fender. Remember that the fender curve has to match the door curve!

FRM

FR Millmore

Looks like the previous repairer has installed a plate over the dent. Note the rectangular shape of the dent from inside, and the deliberate recessing of the dent edges to accept the covering sheet. All this is speculation, as I have not gouged down into the bondo yet. It will probably make it much harder to beat out straight.
Art Pearse

I wondered about that rectangular shadow - hard to see what the contour is in the pic. It will be a challenge! Damned barbarians. Not much to do except carefully cut the overlay off, grind the residual welds off, and go from there. Also wondering about the several spots scattered around. It is always much easier to straighten things right the first time, than to clean up after stupidity and incompetence.

FRM
FR Millmore

Following on from my now archived post - I bit the bullet and removed the fender bulkhead as stage 1 of the operation to repair the PO crash damage and horrible "repair". Unfortunately I had to take it out in 3 pieces due to some nicely concealed spot welds. I have now straightened out the bent and creased bulkhead parts and am halfway towards reassembling them. Here are the bits:

Art Pearse

And now the current state. Next step is to remove the bondo from outside and see what's under it - maybe this should have been step 1 !

Art Pearse

Did I take on too much? I have the bulkhead finished and it matches the door profile. Now for the hard part, flattening out the PO's work. I cannot imagine what he was doing, there are braze spots all over, some surrounding the dented area and some inside. some are very thick as if to fill in holes. Maybe he tried to attach a new outer plate.

Art Pearse

The bulkhead

Art Pearse

That is one ugly fender, you're braver than I am. Nice work on the sills and bulkhead though, best of luck to you!
L Wheeler

Art-
Well, you are pretty well screwt!

Brass is what is called "hot short" when it is approaching melting temperature. The lower melting components act as a sort of lubricant along the crystal boundaries of the higher melting parts. This causes the whole structure to be weak, and it simply falls apart under even small stresses.
When used as a brazing alloy, this delightful property is transferred to the steel as the brass penetrates along the grain boundaries. It produces the same "hot short" quality, and it also means that it is impossible to remove the brass from the steel, since it has penetrated into the crystal structure. Any stress applied to hot brazed joints will cause cracking, and any stress already in the material can cause the same cracking. For instance, simply reheating a brazed joint can cause failure, unless you add brazing material to effectively make a new joint.
Cracking can also occur if stresses are applied to a cold joint, but these are usually from preexisting cracks formed when hot. They will appear if you attempt to cold work a brazed section.

The net result of all this is that you cannot remove the brass, you cannot hot work the brazed area, and you would have very limited success in cold working the area. I suspect that the mess you have was an attempt to hammer out sections which had already been brazed, which caused cracking, which was then unsuccessfully repaired by brazing, and so on in a great kerfuffle.

What you have to do us cut out ALL brazed areas and replace with uncontaminated steel. Under OA welding heat, I've found brass a good half inch away from anything I could see when cold and clean, and it will cause trouble after welding and hammer finishing.

Regarding steel, there are some important variations. Body panels should be done in "DQ = drawing quality" sheet, also known as "SK" (silicon killed) or "AK" (aluminum killed), referring to how the metal was produced. This stuff is very different from common cold rolled steel, and is a joy to work with. If not available from a metal or body shop supply place, panels cut from older cars (pre modern hi strength steel, c 1980) are a good source. I like roof panels. I've also found that any product that involves severe cold forming but requires good paint finish is frequently DQ. Non DQ steel forms a sort of grain pattern which shows through paint, so things like refrigerators or washing machines may be good. I came upon some old school lockers which were perfect, and have picked up scrap from new production in junkyards which handle industrial scrap.

FRM
FR Millmore

Well, that is fascinating and daunting at the same time. I always thought that brazing to steel was legitimate - high quality bicycles are (used to be) made that way. The extent and depth of the dents means that if they could be hammered out, the resulting metal is going to be over-stretched. Looks like I'm going to patch it as you say.
Art Pearse

Brazing of steel is perfectly legitimate, so long as no stresses are set up while the parts are in the critical temperature range. Jigging is the vital point; parts have to be free to move as they cool, but be retained in position until then. Bicycles, motorcycles, and the beautiful and very light front subframes of XKE are brazed, but you cannot rework any of them without very special care. It seems that you should be able to heat up a brazed socket joint, as on many MC, and have it come apart like a sweat soldered pipe, but what generally happens is that the tube breaks like glass as soon as you pull on it when hot. You then have to machine out the stub to fit a new tube.
Point here is that you cannot hammer the dents out, and you cannot heat it to anneal or shrink anything, so patching is the only cure. Given the work you have done, I am certain you will be OK once you get past the psychoblock. The patching will be much quicker and easier than messing with the mess!

FRM
FR Millmore

Be careful when brazing as I've had problems in the past where body filler doesn't seem to want to adhere to brass and starts to come loose. Make sure you clean the area thoroughly after brazing and sand for a good bite for the filler. Brazing does work well if you're using lead as a filler though.
B Young

The borax flux used in brazing can be troublsome too. It is as hard as glass and the very devil to remove, and like glass, will not accept paints and other coatings readily.
Lindsay Sampford

I noticed when chipping away at the bondo that it came off easily from the brass.
Art Pearse

This thread was discussed between 15/06/2011 and 23/07/2011

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