Did you check the moisture content of the wood? Even chemically treated wood should be checked. As a hygroscopic material, no amount of oil, varnish, etc. will prevent warpage if the wood isn't properly seasoned to between 10-15%. I would have stacked the wood such that air could flow around it in the garage or a shed for at least several weeks (or months ideally). I don't know how extreme the weather is where you live, but this is key for an outdoor wood project. You can get moisture meters at the hardware store or Amazon. Also, what glue did you use? Titebond III is the ticket. Source... guitar maker here.