A thin-film coating applicator assembly is disclosed for coating substrates in outdoor applications. The innovative thin-film coating applicator assembly is adapted to apply performance enhancement coatings on installed photovoltaic panels and glass windows in outdoor environments. The coating applicator is adapted to move along a solar panel or glass pane while applicator mechanisms deposit a uniform layer of liquid coating solution to the substrate's surface. The applicator assembly comprises a conveyance means disposed on a frame. Further disclosed are innovative applicator heads that comprise a deformable sponge-like core surrounded by a microporous layer. The structure, when in contact with a substrate surface, deposits a uniform layer of coating solution over a large surface.