Continuous thin zeolite NaA films have been synthesized on macroporous α-alumina supports subjected to a simple pretreatment and without seeding. By modifying the supports with a cationic polymer and even with glucose or polyethylene glycol and subsequent calcination at 673 K, a substantial improvement in terms of layer continuity and crystal intergrowth is observed compared to coatings prepared on unmodified supports. We attribute this positive effect to a fine interplay between the presence of a monolayer formed by carbon–oxygen species, mainly of the carboxylic type, on top of the calcined supports and to the negative net charge of the surface under synthesis conditions. The resulting double layer of solvated Na+ ions on the support would act as nucleation promoter and formation of the NaA thin film. This new method presents several advantages and may form the basis to scale up membrane synthesis in view of the simple and cheap pretreatment together with the elimination of pre-seeding steps.