There are several things to learn here.
1. Subbing was applied to the film as it ran over a large metal roller which kept it stiff and flat until the solvents evaporated. Thus, the film was flat. Doing it by hand loses this advantage. The support can develop wrinkles or kinks.
2. Latexes are often used as subbing layers.
3. Many supports are just bombarded electrostatically, and then plain gelatin is laid down on this charged support. It adheres wonderfully. Sometimes a latex layer is laid down instead of gelatin.
4. Metals in the subbing layer can be used, but many cause severe defects such as total fog. Tin and Iron are among them.
5. Using formalin is ok but remember it is a gas in water and is also a carcinogen.
I just buy pre-subbed film support and avoid this hassle. Or, I use plates.