Just for the record you could take the pieces to a body shop and have them shot in 2k urethane. You can seal them up good and make them pretty impervious to the elements that way. If you need product suggestions for a sealer
www.tcpglobal.com 's house brand is fine. I do this day in and day out for my day job.
cabinet (or cabinet finishing) shops can also spray it but it tends to be lacquer or conversion varnish. It'll seal up great but may not be as durable as a 2k. I remember as a kid we dad did a couple wood beds for car club friends (he has a cabinet shop).
Pine is soft so no finish you put on it will keep it from dinging up.
Oil finishes are ok, good for keeping the wood from drying and splitting but not a ton of protection from the elements. Look at TruOil. It was developed for finishing gun stocks and is essentially a polymerized tung oil so it actually dries, can be recoated till your hearts content (it will build up a finish) and has decent weather protection.
Otherwise deck finishes would probably be fine (i.e. johnsons water seal or whatever) and pretty easy to do.