Protein folding is crucial for cell homeostasis, and disruptions in these mechanisms can lead to inappropriate protein aggregation into deposits. Such deposits form prominently in neurodegenerative diseases, including (polyQ)-expanded Huntingtin protein in Huntington Disease, and mutant superoxide dismutase 1 and dipeptide repeats (polyGA, polyGR, polyPR, polyPA, and polyGP) in the C9orf72 gene found in some cases of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. While the deposits comprise of different proteins in different diseases, the mechanisms by which they arise and confer cellular pathogenesis remain unclear. In particular, some of the protein deposits display different patterns of miscibility with other proteins, raising the prospect that co-aggregation specificity could play an important role in driving the mechanisms of dysfunction. Here we describe our findings exploring the mechanisms by which different proteins mix into different protein deposit phases and how they form at interfaces. With fluorescence microscopy we found that proteins of different composition range in being completely immiscible with each other, as well as having variable patterns of “wetting” of the surface of the aggregated species. We have also found that the sequence grammar can contribute to phase separation properties of unfolded proteins. Certain residues can function as “stickers” for phase separation that act as cohesive forces for intermolecular interactions. Determining the mechanisms behind different aggregating and co-mixing species is key to understanding potential therapeutic strategies.