While fingerprint evidence is not as fragile as some people think, you do have to handle it appropriately. If it's raining, don't be too quick to move a vehicle before lifting prints. Process at the scene. If you tow the vehicle in the rain, the grit and grime from the wrecker will act like sandpaper, and you’ll lose the prints. Also, if you let the surface dry out, water spots will form and interfere with the quality of the fingerprints. On the other hand, oil and water don’t mix, so the fingerprints will remain intact on a wet vehicle.

You have two options for developing and lifting latent fingerprints from a wet surface: SPR and Wet Print. SPR is a dry powder that you mix with water and Photo Flo. Once you spray the solution on the wet surface, the SPR adheres to the latent fingerprints, allowing you to see them. Because SPR sprays on grey, you may have to use multiple applications to get a black print. Wet Print is a premixed solution. Spray it on, then rinse with water, and the prints will be right there. You only need to apply Wet Print once; you get a black print the first time. One drawback to both these products is that they can’t be used in temperatures below 32 degrees because they are water-based. So, in winter weather, you’ll have to let the car or other surface thaw out before you begin processing.

From: Adverse Conditions by Dick Warrington