I don't think anyone has been using CA as a finish long enough to have any long term experience with wear and durability. We have benefitted from the fact our pens are generally treated like a more expensive Bic by most people. The average pen is thrown in a drawer when the cartridge runs out and that is the last we ever see of it. However, we are starting to make fountain pens and others that sell for more than $200. This means that people will be using their pens longer and ecpecting more from them. Whether anything we are using for a finish, or whether any of the platings that are available to us are up to this longer life are questions that will be answered in the next couple years.
Yellowing and becoming brittle are characteristics of aging for anything that we can put on a piece of wood. It is too soon to know for sure whether CA glue is any different. However, the results so far appear to be very favorable for CA glue as a pen finish.
My guess is that using Boiled Linseed Oil (BLO) in the application is preferrable to using CA glue alone. Any residual oil molecules in the cured finish will act as a plasticiser, thereby preventing it from becoming brittle with age. This happens with all other finishes, and I see no reason why it wouldn't do the same thing with a CA glue used as a thin film finish. The lesser gloss from the BLO application of the CA tells me that there is indeed some retained oil in the finish.
I also suspect that some form of a lacquer will prove to be the most durable pen finish. Lacquer was used over Ebonite and other materials for the old fountain pens from the 1930's. Many of them are still looking good after years of use and being knocked around in a desk drawer. I see no reason for the same lacquers not being as durable on a modern pen pen that we make from wood.
The polyurethane finishes like Enduro are in the same boat with the CA glue. They are looking good, but it is really too soon to tell.
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