Gum solutions are formulated to protect a negative-working photopolymer imaging layer coated on an anodized aluminum substrate that has undergone a post-anodic sealing treatment with inorganic phosphate and inorganic fluoride, followed by development. In various embodiments, the developer and gum solution have different compositions but contain at least one polycarboxylic acid—which may be a polymer—that beneficially desensitizes the surface after the unexposed photopolymer layer is removed.