Bondo Mold Making for Pewter Casting

Recently I was at a friend's house (Philippa Montague) going through a "how-to" session on pewter casting.  We were planning to attempt making molds in both soap stone and Bondo.  I had heard about the Bondo mold making process and had experience using an a Bondo mold for casting coins.  I had never personally experienced creating a Bondo mold.  Bondo works well to replicate a small object in high detail so I decided to put together this step by step set of instructions as I tried experience the process myself.

This post will not be covering the stone molds or using the finished mold.  It will focus specifically on the process of making a Bondo mold.

What is Bondo?

As described on the company website: http://bondo.com

Bondo® Body Filler, Pint Can (Net Weight 14 oz), 261

This industry leading body filler quickly and easily fills dents, scrapes, chips, cracks, and rust holes. Because it is specially designed to be non-shrinking and permanently durable, this reliable filler is great for a wide variety of automotive, home, and hobby projects. Experience the accomplishment of completing a repair or restoration with Bondo® Body Filler.

Extremely versatile filler perfect for a wide range of projects and applications

Cures fast and shapes in minutes allowing same day completion of repairs

Great for use on wood, metal, aluminum, fiberglass, masonry, and concrete and compatible with all paint systems

Spreads easy and sands in only 20 minutes

Includes cream hardener

Fear not, although the container says nothing about using Bondo for making a pewter molds it works very well.  As long as you cast at a reasonable temperature the molds can hold up for thousands of pours.

Materials:

• Bondo (Body filler with cream hardener)
• Silicone spray
• Container to mix Bondo in
• Spatula for mixing Bondo
• Lumber for making frames
• Screws
• Electric screw driver (optional)
• Modeling Clay (optional)
• Waxed paper (optional)

Method:

1.    Object to cast

1.1.  The purpose of this exercise is to create a mold which will allow you replicate something using pewter as your medium.  You may decide to copy a coin, a small belt buckle, a pilgrim's badge or some other extant item.  You might also consider creating your own object from modern clay or "green stuff".  For the purposes of this exercise it doesn't really matter what your source material is and you can assume that I will be copying a small coin which may be used for a local SCA gambling event.

1.2.  This project will consist of a two part mold but the concept could be used on more complex multi-part molds.  Each section of the mold is formed separately so it doesn't really matter how many pieces you would like to add.  To ensure that the object is easy to remove make sure that there are no sharp edges which may snag on the mold once the piece has been cast.

2.    Build Frame

2.1.  Base - The base of the frame is simply a board thick enough for the purpose.  There will be three basic levels to the frame.  The layers will be referred to as the base, level 1 and level 2.  For this project a 1 inch thick piece of pine will suffice.  If your wood has rough or very porous surface you may decide to cover it with waxed paper to make the removal easier. You might also consider using a section of a laminated counter top.  The laminate will reduce the likelihood that the Bondo will stick.

2.2.  You can think of the frame as a bi-level house.  The base is the foundation of the home and the levels are the first and second story of the house.  The base, or foundation level needs to be very smooth and even.  Level 1 and 2 are are basically identical in their construction although they will NOT be permanently assembled.  These pieces should be cut and predrilled so that they can be easily assembled and then disassemble as you use the frame later.

NOTE:  The frame should be symmetrical.  You will be casting your first level, flipping the piece and inserting it back into the frame.  If the frame is not completely symmetrical the piece will not fit properly back into the frame once flipped.

  

As you can see in the next image the screws are counter sunk.  That should make it easier to use a spatula to smooth out the Bondo along the surface of the mold.

  

 

3.    Insert object

3.1.  Attach the base to the first level of the frame.  Keep the electric screw driver handy as you will need to take the frame apart after the Bondo has been poured into the mold.

3.2.  At this point you may choose to create a sprue out of clay.  You will need a sprue on the finished mold.  Bondo is easy enough to carve later, but if you create a clay sprue you can simply remove it and not have to worry about carving the opening later.  You will also conserve a little Bondo in the process.

4.    Spray with silicone

The piece which you are going to replicate may be out of various materials.  Keep in mind that Bondo was originally designed as a auto body repair material.  It sticks very well to metal.  If you are casting an object made of metal the Bondo WILL stick to it.  To allow the piece to be easily removed later the piece will be sprayed with a silicone spray.  The inside of the frame will be coated with a thin spray of silicone as well to ensure that the Bondo does not stick to the frame or base.

5.    Mix Bondo

Following the instructions on the can you can now mix up a batch of Bondo to create your mold out of.  You will not be able to use this batch for both parts of the mold so only mix up enough to fill the space in your first level of your frame.

 6.    Fill Mold

6.1.  The consistency of the Bondo will thicken the longer you wait.  To ensure proper coverage over your piece and therefore sufficient detail on the finished casting the piece should be covered thoroughly.  It is helpful to use a small brush or spatula to press the Bondo around the piece.  Try not to mix in bubbles into the material as you mix it as this may cause imperfections in the finished casting.

6.2.  Pour the rest - Once the piece is covered sufficiently you can spread the rest of the Bondo into the frame.  The neatness of the surface is not highly critical as this will be the back of the mold althought it needs to be flat enough to be flipped and inserted back into the mold.  The more smooth you can get the finished mold the easier it will be to use later.  Spread the Bondo and scrape off any extra so that it is even with the top of the frame.

6.3.  Let set - You will need to monitor the material closely at this point.  Depending on the temperature of your work location and how much hardener you mixed into the Bondo the set time may vary.  You do not want the Bondo to harden completely.

7.    Remove frame

7.1.  The Bondo mold needs to be removed from the frame as soon as it has become slightly rigid.  Remove the screws from the frame and separate the Bondo from the base as soon as material seems to become slightly firm.

8.    Remove object

8.1.  Remove the object from the mold gently.  The Bondo should still be pliable enough to allow you to remove the piece but firm enough to hold it's shape once the piece is removed.  If you wait too long you may find that the piece cannot be removed without damaging the Bondo mold.

9.    Let harden

     9.1 If you are making a one sided mold you can consider your project done.  You can use a piece of soapstone as the back and try casting with your mold.  This project, however, will be a two part mold.  Before proceeding to the next step the first half of the mold should harden completely.   

10.  Insert mold part 1 with object into frame

     10.1  Assemble the level 1 frame again.  Once the mold has hardened completely you can replace the object in the mold.  The first half of the mold should be placed back into the level 1 frame face up.  Since the object is face up this time you will be able to cast the back side of the piece using the second level of the frame.  Note: If you used a clay bit for the sprue you will need to insert it again as well or the second layer of Bondo will fill in the sprue.

11.  Add frame level 2

     11.1  Assemble the second frame on top of the already constructed first layer.  

12.  Mix Bondo

     Following the steps you previously used you can mix up another batch of Bondo.

13.  Fill mold

     13.1.  Using the same techniques as previously used cover the piece, pour the Bondo and then smooth off the surface of level 2.

14.  Remove frames and object

     14.  As soon as is possible remove both frames and the piece using techniques previously discussed.

15.  Let harden

     15.1. If you haven't already accounted for your sprue you may decide to carve a sprue while the Bondo is soft.  Once cut let both pieces hardened completely before using them for casting.

Optional Step - Sanding Mold 

     You may find that the object you are replicating is thicker than you would like your copy to be.  Say, for example you are casting coins.  You would like to minimize the use of pewter used for the finished coin. If you sand the face surface of the Bondo mold the finished piece will become thinner.  Bondo was originally created as a auto body filler so it sands very easily.  Make sure that your finished molds still have  decent seal when placed face to face before casting.

16.  Cut Sprue

     16.1. If you haven't already cut your sprue while the material was soft you can easily carve it even after the Bondo is completely hardened.  Bondo carves much like soapstone so any wood carving tools will aid you in removing a sprue section.

  

FINAL NOTE:

I have used this process to make a replica of something which I plan to use as part of a gift.  Once the gift has been delivered I intend to add photos of the actual materials instead of the graphic images.