The number one mistake officers make is over-processing the latent fingerprint. First rule: use less powder than you think you need. Because of this, I personally never place the brush directly into the powder jar. Instead, give the print powder jar a quick shake before opening the lid. Remove the lid and place the fingerprint brush inside the lid. The lid has enough powder to load your brush. Now the brush is ready to develop the latent print.
I gently spin the brush in my hand and apply light pressure on the surface to develop the latent print. Too much powder will overdevelop the fingerprint by filling in the ridge detail, while too little powder won’t provide enough contrast. Also, limit the number of brush strokes while developing the fingerprint. The brush itself can actually damage the print.
Once the latent fingerprints are developed, it’s time to collect the evidence. Depending on your agency and the type of case, the prints can be photographed before collection. It’s critical to apply the lifting tape smoothly and evenly.
My best tip is to use a credit card. I secure the lifting tape about an inch from the edge of the print. I then use the edge of a credit card to gently press the fingerprint lifting tape onto the surface. I do this in a smooth constant motion. This ensures complete adhesion between the tape and the powder, while eliminating air bubbles from being trapped under the tape. Gently pull the tape off of the surface and secure the edge of the tape onto the glossy side of the fingerprint backing card. I use the credit card again to carefully press the tape onto the card.
Using a pen or marker, I date and initial the edge of the lifting tape on the backing card. I recommend drawing a simple diagram and writing a short description of where the fingerprint was lifted. Finally, it’s important to detail in your report your exact actions on scene.