For a nice knife cut effect you need a small syringe or an eardropper and a length of aquarium tubing. Tape the tubing to the back of the knife (away from the camera). If you cut off an inch or two from the tip of the knife or cut a small semicircle in the blade it gives the illusion that it?s in the flesh. Squeeze the syringe as you draw the knife across the hand and the blood will flow, looking like it's coming from the cut.

A scar can be made using Derma Wax. Place a bit of the wax on the person and spread it out to the desired length with a wooden sculpting tool. Blend the edges into the skin using baby oil, pat dry and then cover with make up.

Build up the wax a bit, make a jagged cut with a wood sculpting tool and add blood for a nice looking cut.

Gelatins are used in a similar way. The gel is heated then applied and sculpted on the actor. Of course, be careful about how hot you let the gelatin get...

Liquid latex can also be used. Stretch the skin, apply a very thin coat, let it dry and then release the skin for a wrinkled look. Layer very thin pieces of tissue paper with liquid latex to create pealing skin.

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I was thinking about this the other day, and I came up with an idea, but I'm not too sure how well it will work. Let me know what you think.
Fill a condom with some theatrical blood, since condoms break very easily. Wrap a think piece of cloth around your actor's part of the body where the knife wound will be. Lay the condom exactly where the cut will be and then paint over it with a few layers of latex paint that matches the skin tone of the actor.
This method is made for a zoom up view of the cut, while it is being inflicted. Since you are using latex paint and a condom to hold the blood, it should look at if it were a real cut, since it would look like real human flesh separating.

Sounds cool. My version of it was to tape condoms full of blood to an actor's chest with fishing lines and hooks also taped to the condoms under the shirt. Then at the right moment pull the fishing line and it spills.

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