body work block sanding filler question

Over the weekend I had a car blasted to remove paint.  It did some pretty serious damage to my panels.  The trunk lid is dead flat, or at least it is suppose to be.  The underside has ribbing to give it stiffness.  After sanding down the lid to prep for sealer, you could make out the entire outline of the ribbing below.  I'm guessing the blasting stretched the panel over the ribs.  Its not flat.  If you lay a straightedge across it, you can see daylight in between the areas where the ribs are.  If I skim coat this and block sand it flat again, will the filler stay there without cracking?  Whenever you open and close the trunk, the lid flexes.  Whenever you close the lid, air pressure from inside the trunk causes the whole lid to wiggle.  Will the body filler be able to move with this type of deformation.  I've spot filled low spots in a lid before with no problems, but never had to skim and block and entire panel.  Tons of videos on line of people skimming and blocking fenders and other curved panels that don't move.  Nobody doing a dead flat panel.  99% sure hood is trashed.  The warping on that thing is severe.  Roof is bad as well, but I'll try and fix that with heat, hammer and dolly.

Ook. I'm here more to hear what more experienced folks say, but it sounds to me like the blaster heated the panel and warped it. I'm going to guess it's not so much that it stretched the panels over the ribs, as that the ribs kept the parts of the skin they were in contact with from getting as hot. Says the guy who's read a ton and done very little...

Fingers crossed NOHOME chimes in that some clever use of a shrinking wheel will sort  you out, but I figure trying to go back to dead flat is about the hardest thing one could attempt to do...

What type of media was used in the blasting? Soda definitely shouldn't have done this. 

Also get a heat gun and dry ice and see what you can get to return to form before doing any filler work. 

Tough work, but if I were going to attempt it, here is what I would do. You need a shrinking disk and a long board sander. Coat the panel with a guide coat of paint, and then lightly sand with  the board sander. The places where the guide coat is scuffed off are your high spots, and the places where the guide coat remains are your low spots. Attack the high spots with your shrinking disk.

Repeat the whole process until the panel is flat or you drive yourself crazy.

- chris r.

In reply to intrepid :

Based on the initial description, I think the deformed parts are the low, with the bits over the braces at normal height... I do wonder working on them from the bottom with a shrinking disk could work.

This of course also makes it really hard to work the long sanding board into it.

When I get to a real keyboard I will have a very long post for you. 

The short story is that the damage has nothing to do with heat warping the metal. It is a stone cold vajajay to fix.

Pete

Decided to find another trunk lid and start fresh.  Found a Webasto sliding roof for the top, so who cares if the roof is warped at this point.  I'm about to cut a big hole in it.  The Webasto is a cool addition even if its going to be expensive to ship and recover.  Still trying to figure out what to do about the hood.  TR8 double bulge hoods are hard to find, and even more expensive to ship.  I've resigned to the fact that this one is not salvageable.  

I'm sorry, that blows.

Still curious to hear NOHOME's explanation of what happened...

When you  sandblast a panel, you are hitting the surface with high energy particles. If the particles as hard enough and have enough velocity, they actually move the surface layer of metal. Think of this top layer as being compressed by the media.  Believe it or not, heat has nothing to do with the metal going all wavy.  

When the top of the metal is squished out, but the bottom is not, something has to give.  So the surface changes shape by introducing waves that accommodate the new surface area. Usually the waves are such that they pop in and out with a bit of finger pressure, hence the name "Oilcan Dent" for those old enough to recall such things as an oilcan.

 The only way to fix is to shrink a particular spot of metal that will release the oilcan. But you have to find the magic spot cause it is not in the dent itself.  Push the dent out to where it should be, then while tapping on the dent ( but not pushing it over the edge) go around the area outside of the oilcan and press down with your finger on a spot. You will find a spot that "locks" the oilcan in place. That is where you want to shrinking with the shrinking disc or a torch.

Unfortunately, it is not all as easy as it sounds and the panel is going to require a lot of hammer and dolly work to restore the shape. When it is all said and done you are going to need a skim of mud over the panel to make it perfect. How much mud you need depends on how much patience you have to shrink and beat the panel.

You can see the oilcans in this pic after I ran the long blocks over the panel

Here we are after at least 20 hours on the shrinker with a lot of dolly and slapper work to form the shape. Not perfect but we are now looking at waves of 1/16th of an inch and a panel that does not pop in and out when you touch it; you dont want your filler to be what locks in the shape.

And then you end up doing this

Sand most of it off

Can pretty much see through most of the mud, but it does skim the whole panel. The shadowy black bit is guide coat used to check how level the panel is.

And primed

Keep in mind that most of the car had the same damage and you get an idea of how much I hate bad sand-blasters. Pretty sure that they destroyed someones dream car and that is why I got it cheap. Always buy the new panel before you fix blast damage.