Basically, there's no way to put a permanent waterproof finish on
exterior wood with a reasonable cost to benefit ratio. At least, I've
never heard of such a product. What you got is as good as it gets,
though sun and rain will eventually break it down. So you'll have to
powerwash and/or sand and recoat every few years. For next time, look
for high quality, UV -resistant oilbased stains for longest
durability. With semi-transparent stains the prep is especially
important to avoid build-up of pigment after multiple coats.