I just left a couple of pieces of oak out on the patio for a year or so and got nice weathering. I have used a product called Weather-It for small projects in bass wood and it should do OK on pine. If you are going to make frames out of pine, you will want to consider cutting away from knots, starting wih wood that is down to low moisture contents or putting up with a lot of wood movement, etc. Usually, you will buy or chew up several times the amount of pine to come up with the same good quality frame that you could have with a hardwood and with a better looking end product.