there is absolutely no need to paint the interior surface of the fiberglass
with an anti mold paint. Fiberglass
itself is not a food source for mold to grow on. What supports the mold growth is dirt getting onto and sticking to the the surface of the fiberglass. Cleaning the fiberglass surface is good enough. But if you can't get it clean then I guess you can use a mold killer on it to kill the spores and then seal the dirty surface with mold inhibiting primer.
If you create a good vapor barrier there is no way for dirt to get in that insulated cavity area. If there is no dirt or other food source and there is no moisture then you won't have any mold growth. It takes water plus a source of food to grow mold. Remember that fiberglass is not food, dirt that naturally contains decayed vegetation in it is the food. Pure clean, washed, sand would not support mold growth. Glass won't, etc.
My blocking is done with cedar which is a naturally mold resistant wood due to natural resins and a high level of tannic acid. I have used some doug fir where I wanted extra screw holding power but it too has pretty good rot/mold resistance. The plywood I have in some areas is a marine trade specialty item made with rot resistant wood.
However some other types of wood when it gets damp are very prone to developing mold and will eventually rot if it stays wet. For woods that are more prone to developing mold you can coat the surface of your wood blocking with a mold inhibiting primer. Just go to the hardware store and ask for some, there are lots of brands of primers that will work. There are both oil based and water based versions. For interior use because of air quality in confined spaces I use water based. It also dries faster.
Today I took down one of the wall panels I put in year before last in the spring and then cut out a section of the insulation. Not even a tiny bit of moisture or condensation staining was behind the panel, no sign of mold but there has been lots of cold, wet weather since I installed it. I took the paneling off that area because I wanted to add some extra blocking as a strong backer plate so I screw in a fitting on the exterior wall.