Sometimes Autobody work requires a more delicate approach than the sledgehammer...
Here comes a tool called a "Slapper". They have been around for a very long time and it is still in use by antique restorers everywhere. There are some commercial versions for sale but they are rather hard to find and pricey for something that is relatively easy to make. They are great for aluminum work.
Here is my version of one of these tools.
I made it at TechShop Chandler, techshop.ws.
Step 1: Gather Some Materials
I made mine out of some scrap. A discarded knotty piece of pine 2x4 and a strip of MDF used for cement forms is what I used. Because I plan on using mine on some aluminum I decided that pine is soft enough not to put extra dents in the material I will beat on. Some of theses Slappers are made out of hard wood and faced with leather. You might want to get some maple or other hard wood not prone to cracking and enough thick leather to cover the convex part of the slapper. Leather is attached with some metal sheet pan head screws, in such a way as not to hit the work with them.
Step 2: Getting Started
For tools, I used the table saw, band saw and various sanders. The hand drill was used to drill a hole in the handle for a lanyard.
First made a template out of the 1/4 MDF strip. Using a pencil, I drew a rough outline that looks like the picture. It needs to be about the right size to fit your hand and have the curvature needed for the type of work intended. Don't make it excessively long, it is not a Club !
Body panel people have a whole library of these tools of different shapes and each with a various radius. make a few drawings and cut them out of the MDF.
The cutout, even in the rough, should allow the user to get an idea about the size and how it fits in the hand. Make sure the handle is big enough so the whole palm can wrap around the handle and provide a comfortable grip.
Step 3: Shaping Template
Take the rough cutout to the sander. The combination Belt and Disk sander is used to easily take material down to the line and create a smooth transition from one curve to another. This is where the most important shape of the slapper: the face, and the handle is mostly done. It is easy to get the face sanded to the right radius in just a few minutes.
The concave surfaces such as the back side, and the areas that make the transition from the handle to the body are best sanded on the Spindle sander. Choose a large sanding tube, the large surface makes shaping faster and the curves smoother. Keep sanding away until you think the tool fits in the hand just right, and the curvature as well as the whole tool is free from band saw marks.
Step 4: Shape the Tool to Size
Once the template is good, take a pencil and trace it to the material you decided to use. Pick a clean area without knots. Defects will cause the slapper to split from hammering into metals. I traced mine onto the 2x4 piece I had.
Save the template it can be reused and it is the basic shape for future designs.
The resulting outline, is then cut out again just like the template on the band saw. Take care to follow the lines closer this time. The material is thicker now, and you will spend a lot more time sanding waste away, than on the thin MDF previously.
Just like the template, take the piece to the belt and disk sander and carefully bring the piece down to the contours outlined by the pencil marks. Use the Spindle sander to get into the concave surfaces and the areas around the handle grip. Make sure all the saw markings are gone especially on the face, and that there are no "waves" left on the face. The face should have a smooth continuous curvature without high or low spots.
Step 5: Shape the Handle
Now that the overall shape is established, we need to work some on the shape and comfort of the handle. Usually beating on body panels is a rather long an tedious job, so comfort is important.
Use a pencil and draw lines outlining the material to be cut away. To get an idea use the dimensions from a large brush or butcher knife that fits in the hand comfortably.
Take the slapper to the band saw again and cut the outlined waste pieces out. Pay attention to the areas around the transition to the body. Get a smooth exit from the blade of the saw. Sharp areas are not easily sanded and put stresses in the wood leading to cracks. Use the Spindle Sander, and again smooth the handle into a nice transition. At this point you can see how it fits in the hand, and you can take off more material as needed.
Use the Spindle sander to also round the handle in the other planes so there are no sharp corners. Don't take the face too round. In fact just touch the edges lightly, just removing the sharp vertices. In the case of leather facied models, the leather needs a nice flat surface for backing.
Step 6: Finishing
Now that the tool is all sanded to shape, you only need to put a hole in the handle so you can put a short loop of string, in order to hang this somewhere out of the way when not in use. It should not be thrown in a heap with other tools as the face needs to remain smooth and free of nicks and dings. Any imperfections in the face will transmit to the panel being beaten on, and defeats the purpose of the whole tool.
If faced with some thick leather, put some leather conditioner on it from time to time, it will keep leather facing supple and it will not crack.
It is worth putting some polyurethane or similar finish on the tool also. Working on cars or with metal is usually a greasy job, and dirt and grease will soak in the tool if the surface is not sealed somehow. It also makes for a neater appearance !
I hope the amateur car and motorcycle restorer out there get some use out of this tool. There are pictures on the internet with various shapes for wood slappers, copy the shapes for your projects.