[special-K]

Mix boiled linseed oil and turpentine or mineral spirits and slop it on. Let it soak and saturate. You can do straight linseed if you don't mind it being oily for a while till it leaches out some. It will make the wood very happy and repel water. Use a pump sprayer and do the underside, too. Rusty metal loves it and it acts like a clear coat on paint or patina. So your bed strips will stabilize with that on it. If it's a funky old truck, another thing would be mix used motor oil with kerosene and brush it on. Mountain folk did this on their barns. It looks beautiful as it leaches out from blackish at the bottom to golden virgin wood color up top. Your bed is weathered, so no virgin golden hap'nin

That open grain could never be sealed with a hard finish...unless it's resin. Hard finishes are for planed and sanded wood. But even then, it's just like paint and will gouge or scratch, which lets water in and under the finish and eventually blistering it away to a big mess that needs to be stripped and refinished. Oil gets into the wood, like stains do, but oil is resilient to water, rejects it. You know what they say, "Water and oil don't mix"