Deck board coatings are not the way to go. I notice they're no longer carried at my local big orange box store, probably out of business.
If the boards are so bad that pressure washing and staining or sealing will not suffice, then they need replacing for structural reasons.
About Trex and similar composite decking; if you live where they will get a lot of sun, be aware that this kind of material gets very hot due to its density, and may not be comfortable to walk on. Cedar is very lightweight and much cheaper in my area. But for low maintenance, composite is preferable.