Dents in sheet metal are not destroyed metal — they’re displaced metal. The surface area is mostly intact. The arrangement is wrong. That’s an important distinction, because it means most dents can be worked back to factory condition without cutting, welding, or body filler — if you understand what’s actually happening in the metal and use the right tools in the right order.
This article walks through the full process: slapper and dolly work to return the metal to its correct arrangement, followed by a shrinking disc to correct any remaining area change and iron the surface flat. No magic. No expensive machines. Just technique.
Table of Contents
- The Project: 1957 Porsche Speedster
- Tools You Need
- Understanding What’s Happening in the Metal
- Step-by-Step Process
- Why the Shrinking Disc Comes Last
- High Spots, Low Spots, Stretched Metal, and Oil Canning
- About Wray Schelin
- FAQ
- Get the Right Tools
The Project: 1957 Porsche Speedster
My friend Gary bought this 1957 Porsche Speedster 30 years ago. He had good intentions of restoring it himself, but since he’s in the collector car restoration business, his customers’ cars always came first. He made some progress over the years, but a lot of work still remained.
The series of pictures I’m documenting here are of the rear body shroud, which had an impressive dent on the left rear quarter — inflicted about 35 years ago. I offered to help Gary make some progress on the Porsche, and in return he’s letting me copy the surface information of the whole body. My intention is to capture all of the Speedster’s surface and reproduce the panels in aluminum and steel. Building a complete set of 1957 Porsche Speedster body panels will be the featured project of my monthly 4-day metal fabrication class. But first I have to get the panels in shape in order to copy them.
The sandblasted rear body shroud below shows the dent from the outside and the inside. The dent was approximately 4 inches deep and 24 inches long.
I roughed it out using one of my delrin head shaping mallets on my super beater bag — 150 lbs of sand in that beast. My former student Chris stopped by and I roped him into helping me hold the rear body shroud while I roughed out the dent in about five minutes.
Tools You Need
You don’t need thousands of dollars in equipment for this. There is no magic tool, and you don’t need five-plus years of experience. What you need is good information, good eyesight, feel, and patience. Here’s the full tool list:
Shaping Mallet
Used for the initial rough-out when a dent is severe. A delrin-head mallet on a heavy sand bag can push an extreme dent back to a workable rough shape in minutes. The bag I use weighs 150 lbs — it doesn’t move when you hit it. On the Speedster’s 4-inch-deep dent, a former student held the panel while the mallet roughed it out in about five minutes.
Slapper
A flat spring-steel striking tool, wider and more forgiving than a body hammer. Slappers distribute force across a broader surface with each blow — less risk of localized stretch, more even correction across the panel.
The slapper I use I made from a Duesenberg rear spring. It’s the perfect size and I use it far more than body hammers. I’ve yet to see a commercially made slapper with all the right features. I rounded all the working surface edges so it won’t scar a panel if you mistakenly strike at an angle.
Dolly
A hardened steel block held against the back of the panel while you slap the front. The dolly doesn’t just support the metal — it actively pushes low areas up while the slapper drives the highs down.
The dolly I use is a Herbrand Tools dolly — they’ve been out of business for years. I bought it in 1967 from the Herbrand tool rep who came to my grandfather’s restoration shop weekly. It became my favorite body dolly. I use it about 90% of the time. It has a low crown on one side and a medium crown on the other, with very useful shapes on the ends and sides as well.
Magic Marker
Use a 5/8″ wide felt magic marker. I use markers instead of spray dye because they work better. The marker serves two purposes: it shows you the surface map (highs go shiny when filed, lows stay black), and it lubricates the shrinking disc so it doesn’t gall the metal.
Without the marker the shrinking disc will scratch the surface and cause galling. With the marker you won’t have any problem — the disc also builds up a scum on its surface which further protects against galling. If you ever see sparks when using a shrinking disc, you’re galling the metal. Stop immediately and sand the buildup off the disc.
Fine-Cut File and Sanding Block
A German-made Pferd brand super-fine 20-teeth-per-inch file dragged lightly across the panel reveals high and low spots by cutting only the highs and skipping the lows. I rounded the edges so they don’t bite in. Little to no metal is removed — it’s a reading tool as much as a cutting tool.
Also useful: a Baltic birch plywood 6″ diameter sanding disc with 220 stik-it paper on both sides. The round shape works on most panel shapes. The file works slightly better, but both do the job.
9″ Shrinking Disc
The 9″ shrinking disc mounts on a 9″ angle grinder using a central hub support, a 9″ backing pad, and then the disc. The nut has a 7/8″ neck and a large flange — when mounted correctly the nut sits below the working surface of the disc. The disc is used running flat on the panel.
The concept goes back to at least the 1930s when bodymen used dull flex-back grinding discs as surface friction heaters. Master metal shaper Scott Knight created the first stainless steel shrinking disc in the early 80s. I came up with the smooth and safe style shrinking disc and have been selling them worldwide for more than ten years. I also created the first smaller-diameter shrinking disc for small grinders. My discs are available in the tool section of this website.
Understanding What’s Happening in the Metal
Sheet metal is exactly like clay, just more stubborn. Instead of squeezing it in your hands, you persuade it with the slapper and dolly. I separate dent work into two concepts:
- Arrangement change — 99% of removing a dent is returning the panel to its correct arrangement. This is the position of the metal, not the amount of it.
- Area change — Stretching or shrinking changes the actual surface area of the metal. The original denting and roughing out probably added about 1% growth — not a lot, but it has to be corrected.
Think of a cross-section of the panel. At the top: a nice smooth flowing curve — that’s what the panel is supposed to look like. Below it: the roughed-out surface after banging out the massive dent. The slapper bridges two high spots and the dolly positions itself under the low area. When you strike with the slapper, you drive the two highs down and the dolly forces the low up. You’re ironing out the surface ruffles — changing the arrangement.
Once the arrangement is close, the shrinking disc handles the remaining area correction — and irons any final surface waviness flat.
Step-by-Step Process
Most people who work on their own collector cars don’t want to spend thousands on equipment. This method uses few tools and yields perfect results when worked with patience.
Step 1 — Rough out severe dents first
If the dent is more than an inch deep, use a shaping mallet and heavy bag to push the bulk of the dent back. You’re not finishing here — you’re getting to a workable rough state.
Step 2 — Check for severe stretch before slapper work
If the sheet metal is stretched more than 1/8″, deal with it before starting slapper work. Heat that spot to a dull red with a torch, then strike it down with a body hammer. The red-hot metal becomes plastic and flows into itself, thickening and reducing surface area. After torch shrinking, lightly planish with slapper and dolly. The shrinking disc takes it from there. If stretch is under 1/8 inch, skip the torch.
Step 3 — Coat the surface with the magic marker
Cover the entire work area. Every bit of it.
Step 4 — File or sand lightly to read the surface
Drag the fine-cut file or round sanding disc lightly across the panel. Highs go shiny. Lows stay black. Now you know exactly where you are.
Step 5 — Set the dolly under a low area
Place the dolly under a black (low) area. Position the slapper to bridge the shiny (high) areas on either side.
Step 6 — Strike the highs
Strike the highs down with the slapper. The dolly forces the low up. Each strike changes the arrangement. Work systematically across the panel — don’t hammer one spot repeatedly.
Step 7 — Watch for the dolly contact footprint
When a low is only about 20 thousandths of an inch deep, you’ll start seeing shiny footprints where the dolly makes contact — confirmed by a ringing sound instead of a dull thud. Use those footprints to steer the dolly around remaining low areas. That ringing is the panel telling you it’s close.
Step 8 — Re-mark and re-read after each pass
Re-coat with the marker and file again. The black area will have reduced. Repeat until the surface is nearly flat with only minor waviness remaining.
Step 9 — Run the shrinking disc
When slapper and dolly work has the surface close, switch to the 9″ shrinking disc. Run it flat on the marked panel. It will correct remaining area change and iron the surface smooth. Work progressively — don’t stay in one spot. Re-mark and re-read as you go.
Caution: Don’t get carried away with the shrinking disc — you can easily over-shrink. If you do, just stretch it back out again with slapper-on-dolly compression stretching.
Step 10 — Final check
Run the fine-cut file across the marked surface. If the file cuts evenly across the whole area and you’re seeing all shiny with no black remaining, the panel is flat. No filler needed.
When you master this process it becomes genuinely fun. You’re no longer at the whim of the sheet metal — you’re in total control. There’s a real sense of accomplishment when you properly smooth damaged panels without reaching for body filler.
Why the Shrinking Disc Comes Last
A lot of people reach for heat or the shrinking disc first. That’s backward.
The slapper and dolly correct arrangement — position. The shrinking disc corrects area — the actual amount of surface. If you run the disc on metal that’s still significantly out of position, you’re shrinking metal that hasn’t been properly placed yet. You’ll create new problems.
The correct sequence is: rough out → slapper and dolly to correct position → shrinking disc to correct area and finalize.
The disc is also a finisher. It irons. Running it over a panel that’s already close to flat will smooth out minor waviness that slapper and dolly can’t easily reach. It does this by generating controlled friction heat that causes the slightly stretched high spots to contract.
One caution: it’s easy to over-shrink with the disc. If you do, just stretch the area back out with slapper-on-dolly compression stretching — the dolly supports the high side, you slap from the low side to move metal back. The ability to correct in both directions is what gives you full control over the panel. Read more: shrinking disc vs. torch shrinking.
For full setup and technique on the disc itself, see: how to use a shrinking disc to remove dents.
High Spots, Low Spots, Stretched Metal, and Oil Canning
High spots are areas where the metal stands above the correct surface plane. The file cuts them first — they go shiny. High spots may be the result of a previous hammer strike that went too far, or a stress ridge created when the panel was first dented.
Low spots are areas below the correct plane. They stay black when you file. The dolly targets these directly — held underneath while you slap the surrounding highs down.
Stretched metal has more surface area than the original panel geometry calls for. The metal is thinner in those spots. Stretched metal will not sit flat no matter how you persuade it with the slapper — there’s simply too much of it for the space it’s supposed to occupy. It has to be shrunk, either with the shrinking disc or with torch shrinking for severe cases.
Oil canning is what happens when a panel has enough stretched area that it pops back and forth between two stable positions — like the bottom of an oil can. It’s caused by stretched metal under tension. If a panel oil cans, shrinking is the fix. The disc is usually enough for mild oil canning. Severe oil canning may need torch work first. For a full walkthrough of the diagnosis and correction process, read how to fix oil canning in sheet metal with a shrinking disc. See the full metal shaping tools guide for more on reading and diagnosing panel condition.
The Final Result
At my February coachbuilding class I had a chance to do more work on the rear body cowl. As part of the class — in addition to teaching panel making and TIG welding — I demonstrate the slapper, dolly, and shrinking disc technique so students can see the full process live.
This is how the dent looked 30-plus years ago when Gary first brought the car in.
When I last worked on the panel I had roughed out the massive dent and finished the left side, leaving the right side roughed out as a visual contrast. Below you can see the left side condition before the class session.
At the class I completed the left forward side. The results speak for themselves — no filler, no mud, just metal worked back to where it belongs.
I’ll continue using this rear body shroud as a teaching example until it’s completely smooth — factory condition or better. After that I’ll capture the surface information to make copies of the panels.
Looking to remove a dent from aluminum? Check out our comprehensive video on how to do it with simple hand tools: Aluminum Dent Removal Video
About Wray Schelin
Wray Schelin has been shaping metal professionally since the 1960s. He trained in his grandfather’s restoration shop before going on to spend decades building and restoring coachbuilt vehicles — the kind of work where body filler isn’t acceptable and panel accuracy is non-negotiable.
Wray created the smooth-and-safe style stainless steel shrinking disc — a design improvement over earlier grinding disc methods — and has been selling them worldwide for more than ten years. He also developed the first smaller-diameter shrinking disc for smaller grinders. His monthly 4-day coachbuilding classes at the Pro Shaper shop in Charlton, MA are attended by fabricators and restorers from across the country. The 1957 Porsche Speedster project documented in this article is used as a live teaching example — students watch the full slapper, dolly, and shrinking disc sequence on a real panel with a real history.
FAQ
Do I need a 9-inch grinder for the shrinking disc?
The Pro Shaper shrinking disc is designed for a 9-inch angle grinder. Smaller discs are available for 4.5-inch grinders — see the shrinking disc product page for current sizes and availability.
Can I use the shrinking disc without doing slapper and dolly work first?
You can, but results will be limited. The disc corrects area — it doesn’t fix arrangement. If the panel is still significantly out of position, the disc alone won’t get you there. Do the slapper and dolly work first.
What if I over-shrink with the disc?
Stretch it back. Position the dolly under the over-shrunk area, then slap lightly from the opposite side. You’re using compression from both sides to move metal back out. It’s recoverable — that’s the advantage of working metal rather than filling it.
Can this process work on aluminum?
Yes, with adjustments. Aluminum work hardens faster, so you need to be more deliberate — fewer strikes, more reading. Torch shrinking requires more care with aluminum due to its lower melting point. See the aluminum dent removal video for a walkthrough.
How do I know when the panel is done?
Run the fine-cut file across the marked surface. If it cuts evenly across the whole area and you’re seeing all shiny with no black remaining, the surface is flat. The marker and file don’t lie.
Do I need body filler after this process?
No — that’s the point. Done correctly, this process brings the panel back to metal-to-paint condition. Filler is for shortcuts, not for properly worked metal.
What’s the difference between a slapper and a body hammer?
A body hammer concentrates force in a small area. A slapper distributes force across a larger area — more like a flat whip. That makes it better for dent correction because you’re adjusting broad sections of metal rather than hammering individual spots. A body hammer creates more risk of localized stretch.
Get the Right Tools
The process described in this article is repeatable by anyone with patience and the right information. The tools don’t have to be expensive — but they have to be right.
Pro Shaper shrinking discs are the tool Wray uses in his own shop and teaches in every class. Smooth and safe style, stainless steel, available in 9-inch and smaller sizes.
[…] To see the slapper, dolly, and shrinking disc working together on a real dent repair, read How to Remove a Dent. […]
[…] Running the shrinking disc on a panel still significantly out of position doesn’t give you full control. Correct the arrangement first, then use the disc to address what remains. For a full walkthrough of slapper and dolly technique on a real panel, see how to remove a dent from sheet metal. […]
[…] removal, you’ll typically do more slapper and dolly work before reaching for the disc. See how to remove a dent from sheet metal and how to use a shrinking disc to remove dents and waves for the full […]
[…] For a real-world dent repair walkthrough showing the full process from rough-out to shrinking disc finish, see How to Remove a Dent from Sheet Metal. […]