User login

You are here

Heat shrinking panels using a flapper disk on the grinder, removing dents etc

Probably not quite a panel beating how to, but this article might give you some idea into panel beating and 'alternative' methods, recently i've moved back to using a oxy torch to heat shrink becuase i dont like the friction the shrinking disc creates - i think it removes too much steel..

But dont let me stop you from expirimenting in your own backyard, this is one of the best things you can do to learn some basic panel repair!

Dont do this on your pride and joy without a lot of practice, and no im not a pro, just a backyard beater so i take no responsibility :D - I use the "grain" analogy very loosly, you should find out more about sheet steel and steel composition as this will give you a better idea about how it sits - people generally use an oxy torch and something to quench the area with todo shrinking, this is just a trick i've taught myself can be used where you dont need a lot of shrinkage, or a little shrinking over a larger area - with all this said, you should also read this article on heat shrinking from someone who has a bit more of a clue / expirience than me

I've been spending a bit of time recently fixing up the rear quarter panels on my 1969 valiant coupe ('68 dodge dart roughly), initially i thought the car was pretty good - good enough to spray but i was wrong, while I had filled and smoothed the entire quarter panels, doors etc it just wasnt the right shape, it was all a bit roundy instead of good sharp pronounced body lines.

I'm convinced, sheet steel WANTS to be put back into its original pressed shape

The end product - a happy and straight quarter panel
with just a light skim of filler

Sheet metal basics

Sheet metal is formed from hot steel and generally hot rolled into however thickness is required, metal is made of of grains, kinda of like how you would picture wood grains, but a lot more uniform and less than microns in length

When a spot in your lovely sheet of steel becomes "stretched" it is generally caused by something hitting it, which causes the grains to loosen up and sit more elongated (kind of like sitting further apart except no gap)

Steel grains generally "contract" when you apply heat and let it cool, it doesnt 'shrink' when you apply the heat, but it 'shrinks' as the area cools down, this can be a positive or negative effect depending on what you are trying to achieve, also the speed of which the steel cools defines (A) the amount of shrinking (B) The hardness of the steel (the steel becomes harder/brittle when it is shrunk)

Removing a high-spot using a shrinking disc

Tools I have at hand for most jobs

  • A good long sander

  • Grinder with flap-disc

  • Flat-faced panel beating hammer

  • Suitable dolly

  • A straight edge if the panel is meant to be straight

  • A fairly fine file used to just mark the highs

If you are doing a complete restoration job and there is existing bondo / bog / filler, i generally just sand the entire area to what shape the panel should be, the high spots will come thru.

You need to check to see if the high spot is caused by either

  • Something hit the panel and has caused a stretch in the steel in this spot or ..

  • There is some damage somewhere else that is pushing on the sheet that is causing the high spot

The flap disk, highly abrasive, but if you can dig up a warn out one that lost most of its bite, you're in luck!

In the second case if you dont find the culprit, you will end up permanently "embedding" that damage into the panel by shrinking the un-natural high spot, im not sure how you tell the difference between the two, other than knowing how the damage was done, I am no professional :D

Performing the shrink

If all is well and you can see the high spot, gently rub the area with your grinder+flapdisk across the high spot, the metal should go a light yellow color, into a brown and then into a dark blue, if it goes dark blue you've burnt the steel basically and it's going to be very hard to work with, you want to just heat it up to where it just starts to go yellowish, also keep in mind how much steel is being removed from the panel, if the flap disk is tearing it off you may want to find something similar that is not as sharp

Shrinking hammer - which has a heap of teeth on the front, could also be used, this bites and twists the steel crystal/grains together when it strikes causing the spot to "shrink", to be used with a sand bag or a dolly

The panel should start to cool, let it cool naturally - don't quench it unless you want mega shrinking, which for most applications is unneccesary, you can always repeat the process.

You may wish to tap the heat spot very gently with your panel hammer, this is not used to force the steel back into shape but is more to "bump" the steel and help the grains to resettle at a lower point as they cool down, using your guide or straight-edge ruler, check to see the panel is moving in the right direction, as it cools you should literally be able to see the highspot moving down if you look very carefully with the ruler against the panel.

And that's about it, keep in mind the total cooling process doesnt stop for a few hours, you may feel the panel is cool to touch, but go away and come back in a few hours and i guarantee you it would have moved another 10%, keep this in mind - let the panel totally relax before doing any hi-fill/bondo work.

Other tips..

  • EXPIRIMENT FIRST goto the wreckers and grab a panel to practice on

  • Older steel is more maluable than newer steel - classic cars are easy to work on that newer cars due to the difference in carbon in the steel, however new steel is very strong.

  • You almost certainly will need some filler to smooth it out with

  • The goal is to use as little filler as possible

  • Often people just fill the whole panel with bog to get all the filler as high as all the high spots on the filler, by shrinking the high spots you can greatly reduce the amount of filler required and ultimately a better finish.
  • You can easily over-shrink a high spot - usually by applying too much heat, if this happens you will need to sit there for some time and gently hit the effected spot between your hammer and dolly to bump the crystals further apart again, it should loosen up but it will take some time!
  • You can "pull" the panel in different directions by shrinking spots that arent neccesarily on your high spot, you can also modify shaped "patches" on the car that need more of a bend etc using this method - try shrinking on the side of a high spot to see what will happen
  • Keep an eye on the panel as a whole while you're doing this - you may be shrinking a high spot, but then you're causing a big dip in the entire panel

  • You can use a file to score the panel (never file down sheet steel, it will end up like tin-foil) and then a "slapper" to get it totally smooth again - even further reducing your need for filler
  • Panel hammers are used for "bumping" the steel _not_ for beating the steel, you let the steel move as it wonts you cannot force it unless you really know what you are doing

have fun! expiriment! Another good heat shrinking reference