A convincingly-made fake, deep cut adds authenticity to a crash-victim or zombie costume. With liquid latex and some stage blood, you can turn a few stomachs with a proper laceration. Liquid latex is composed of water, ammonia and rubber, and dries to a solid form on skin. With a little bit of cotton and tissue to build up the swelling around the wound, you can create an impressive fake, deep cut. Most costume shops sell liquid latex.
Paint the liquid latex onto your skin with a cotton swab. Make it the size you would like your fake, deep cut to be.
Pull small tufts of cotton about the size of a small pea from the cotton balls. Elongate the cotton and flatten it out. Press the cotton on the wet latex, forming a long mountain with the edges blended down into your skin.
Use a cotton swab to paint a layer of liquid latex onto the pieces of cotton. Add more cotton and an additional layer of latex if you want the wound to appear larger and deeper. Finish with a final layer of latex.
Use a toothpick to slice a cut into the middle of the mountain. This opens up the fake skin and makes a fake wound.
Cover your fake wound with a cream make-up that matches your skin colour. Paint theatrical blood in the middle of your fake wound, and paint some exiting the wound onto your skin.
Jagged edged cuts are more realistic looking.