Do It Yourself

My girlfriend's deck was built out of untreated pine that was never painted or sealed. I dont see anywhere that any wood is rotted or broken yet, but many boards are cupping from the humidity and temperature changes over the last 4 years. If I buy a few gallons of exterior wood sealant and brush on a few layers on top and bottom would that keep the wood from warping any further and protect it from rotting? Do I need to test that the wood humidity is below a certain threshold before sealing? Where can I get a tool for testing wood dryness? Does sealant brand make a difference? I've loved BEHR paints in the past, they do a great job, but there is a brand called READY SEAL that is much cheaper and I'm wondering if its worth the savings or if there is a reason it is cheaper. All advice is welcome. Thanks