I have also used Titebond 3 exterior glue, but not sure which would actually be better? Glue would probably be a tougher coating; however, the primer may absorb further into the grain.
I think any coating, primer, paint, glue, or varnish will work great and significantly extend the lifespan of...
My preference is always Dunn Edwards Premium Ultra Grip primer. I use this heavy based primer for most applications.
I have never had bonding or bleeding issues with this product.
FYI, the most critical step of installing the exterior fascia boards is to properly apply primer, paint, glue, etc...