Bumping metal

Beginners guide to body hammers.

So you’ve got a dent and want to work it out instead of filling it up with a half a can of bondo, but don’t know where to start. I’ve been there, and still need a little bit of filler now and again!

Go pick up a set of cheapo body hammers from Harbor Freight or any of the other fine Chinese junk retailers. You want a few smooth face hammers, and a few pick ends looking bit on the end, one with a small square point, and one with a square head (Shrinker/bumping hammer.) Along with the hammers grab an assortment of dollys. They come in all shapes and sizes, get as many different ones as you can. A slapper is a plus, as well a few good pieces of wood. A small vice with an anvil back, as well as a seam making pliers are good for flattening out small parts after the hammering is done.

The key is to go slow, no big swings. Let the weight of the hammer do the work, and choke on it as you get started. Big hits leave dents, and that’s what we are trying to remove. Hit only hard enough to see a very slight change in the metal, its like eating an elephant a lot of small bites and then the dent is gone.

This is the splash pan off the bus, I already beat the dents out of the front... oops.

Keep the piece clean, knock off as much paint, dirt and undercoating as you can. Keep the hammer faces and dollys free of pits and rust. Don’t use brake cleaner if you are going to do any welding, as it makes phosgene gas… nasty stuff. I like a twisted wire welding brush for knocking off the dirt, a can of carb cleaner and a razor blade for scraping paint off windows to take off the undercoating.

Start with the easiest, flattest part of the problem area. The flatter you can get the surrounding metal, the easier the big creases disappear. When you first start out flattening the metal, keep the sharpest curve of the dolly under the dent or ding. Work the metal with a small face hammer in concentric circles around the center of the ding… while holding the dolly directly under the lowest point. This gives the metal somewhere to go while it shrinks.

Start here:

#1 is pictured above. Then go to two, three and four.

How To:
Stretching: Hammering with a metal hammer, on sheetmetal backed up by metal. This makes the sheet metal thinner.
Shrinking. Hammer on metal backed up by wood. Any hammer, any wood and the metal sucks towards the hammer blows making it thicker.
Shrinker, a tool that makes shrinking faster. It leaves little pits in the metal however.

This is how it all starts out. This is the section marked #1 above. When you hammer, the dolly will be under the piece held firmly. Hit gently and work up and down along the x's while moving the dolly either not at all, or evenly with the hammer. Not moving, shapes metal faster... so long as the hammer doesn't get far enough into unsupported metal that the metal starts bouncing back. Moving evenly gives more control.
Choke up on the hammer as you start, to see how little force is needed to move the metal.

With picks, do not hit hard or the tip may go through the piece. Don’t hit the center of dents as when you roll the top inside out it makes a hard spot, that takes forever to shrink down. They are good for smoothing out the sides of “mole hills” on the sheet metal.

After picking, the dents will have a flat spot around the sides. Now switch to a small headed hammer and a steel dolly and work around in circles till they flatten out. When they look like pancakes, go to a larger hammer, when almost flat switch out and shrink them with a block of wood or a shrinking hammer. They will disappear like the picture below.

The crease is a tough one, as once a hard line forms and the metal has "Bent" it becomes hard to hammer. It is like trying to flatten out an angle iron, go slow to avoid this. Once the crease is flat switch over to the next detail.





Finished:


The beginning: