Physical Developer is very effective for developing latent fingerprints on paper that has been:

Physical Developer is a chemical solution that is highly effective for developing latent fingerprints on paper surfaces that have been wetted. When paper becomes wet, it can cause the oils and sweat present in latent fingerprints to spread, making them less visible. However, physical developer works by interacting with the components of fingerprint residues that remain on the paper, particularly when the moisture allows for a better chemical reaction.

When the paper is wetted, the moisture can allow the developer to penetrate better and interact with the latent print residues, leading to the successful visualization of these prints. This process allows latent prints that may not be visible through other means to become evident, making the use of physical developer particularly valuable in forensic investigations involving wet substrates.

In contrast, untreated paper does not provide the same conditions for developing the prints effectively because the oils and residues may not be optimized for the chemical interactions that physical developer relies upon. Burned paper would likely have altered the surface and destroyed potential fingerprint residues, while painted surfaces typically have coatings that can inhibit the reaction necessary for effective fingerprint development.