David, have you considered using cedar? You mention that the rails are "exposed", does this mean that they're not painted/stained/sealed in anyway, just the raw wood??? I've built countless decks, outside steps, porch railings, etc. and depending on the price comfort level of the homeowner (or builder), CCA is the number one used material for an exposed surface, and cedar falls second (only due to the slightly more expensive cost of it). If you're only doing railings around a moderate sized porch (say 30'x10'), it's not going to cost much at all for cedar 2x4s or 2x6s to do the railing, and it will weather nicely without rotting. Also consider using some liquid nails when attaching, not only will you get a stronger bond, but it will help to seal the joints so that water doesn't get trapped there.