The procedures here will give you a good idea of what's involved in making flat-backed mold.

You will need...

 


Use WED clay to sculpt or select any other object you'd like to reproduce.
Build a wall about 1" high around the entire object as shown. This will act as a dam in future steps.
Coat entire object with Bullseye Shellac or crystal clear spray. This will seal the object.

Mix up a small batch of Brush-On 40 using part A...
...and part B. You will need enough to coat the entire object.
Use a chip brush or any other disposable brush to apply Brush-On 40.

Work fast and continue to coat the entire object. Brush-On 40 will start to set in about 10 minutes.
Make sure that all areas are completely covered with a nice even coat.
Let Brush-On 40 dry for about 30-minutes. Once it is tacky to the touch we are ready to apply the second coat of Brush-On 40.

Mix up a second batch of Brush-On 40. Add some So-Strong Color Tints to this batch and all subsequent batches. This will help you to differentiate between coats. That way you can see if you miss any spots.
Apply Brush-On 40
Make sure that all Brush-On 40 from previous coat is covered. Let this coat dry about 30 minutes.

Mix and apply 3rd coat of Brush-On 40 and let dry for about 30 minutes.
In the 4th coat you can add some Cabosil to the mixture. This will make a stronger, thicker batch that should be applied only to areas of the mold that may tear.
Apply to all undercut areas...

...Weak areas...
...And around the entire perimeter. Let dry about 30 minutes.
Mix and apply 5th and final coat.

Let final coat dry overnight or at least 6 hours.
We will now prep for and apply the mother mold.
Press tin foil onto surface of the object.

Make sure all areas are covered tightly and make sure area around the object is covered.
Mix up a batch of Plasti-Paste using Parts A and B.
You will need enough Plasti-Paste to coat the entire object with about 1/4 in.

Start coating using a chip brush or other disposable tool.
Continue coating all areas of tin foil.
Work fast! You only have about 10 minutes before it sets up.

Let Plasti-Paste dry about 1 hour. Pull the mold from the work surface and clean out clay. You can wash the inside of the mold to remove all clay residue.

You are now ready to use the mold!