The difference first of all. CA is an adhesive. Some pen makers found that it can be used as a finish. No-one else uses it as a finish.
Rustins Plastic Coating is (according to the Product Data Sheet): "a two-part cold cure finish, based on Urea Formaldehyde resins, plasticised with alkyd. In other words a resin that you have to mix with a hardener before application.
The application methods for CA are well known and there are too many variations on the theme to go into here. If you need more information a Google search will come up with innumerable videos and tutorials.
The application method advised by the manufacturers of RPC is (again from the Product Data Sheet) "Using a large brush or varnish roller, apply evenly and liberally, finishing in the direction of the grain." Obviously the word large can be omitted since we are talking about pens. Before we go any further I have to say that I have not used RPC myself on pens and am not keen on it as a finish on other wooden artifacts but I would imagine that the "dipping method" that Les Elm uses for wipe on polyurethane would work as would careful application by brush or cloth. I found the end results very glossy but a bit "treacly" and unnatural but thinning with the appropriate solvent (Rustin's Plastic Coating Thinners) may minimise that.
The best advice on application methods for pens would perhaps be to PM the person who used it. Most pen makers are happy to share their methods and the worst he can say is no.
Finally, before using RPC you might want to read the Safety Data Sheet and ensure you take appropriate Health and safety precautions.
