Mod Podge is an acid-free brand of glue produced by Plaid. Mod Podge is useful for collage and decoupage as it can be used both to glue something down and as a protective layer over it (instead of a varnish).It is available in a gloss, satin, or matte finish. The company website describes it as a the original all-in-one glue, sealer and finish 1.od Podge is useful for collage and decoupage as it can be used both to glue something down and as a protective layer over it (instead of a varnish). It is available in a gloss, satin, or matte finish.