Cheap/Free Alternative: See nail file option below
Cheap/Free Alternative: None, but you likely already have similar sanding tools in your modeling tool box.
Cheap/Free Alternative: You can often scrape hard-to-reach areas with the tip of a hobby knife blade, just be careful to avoid gouging the part.
Cheap/Free Alternative: None. But as a modeler, you probably already have paint brushes around.
Cheap/Free Alternative: Sunlight (maybe). Theoretically you could do this method using the sunlight, but you would have to brush on the resin away from the sunlight and then take the part out into the sun, by which point the resin will likely have dripped and/or ran off of the area you applied it to.
Cheap/Free Alternative: Many other spray paint primers will work well if used properly. Spray paint brands are one of the most hotly-debated topics in the modeling community, and everyone has different opinions and experiences. I won't weigh in on that here, except to say we've had great success with a number of different paint brands and types by always making sure to prepare our surfaces well and paint in light coats.
Cheap/Free Alternative: Any other super glue or cyanoacrylate will work, you just may not have consistent cure times or precision application. We've also found cheap super glues tend to fog certain finishes and plastics.
Cheap/Free Alternative: Other 2-part epoxies may work just fine, but in BSI's product we have found a consistent, high-quality product and have never strayed.