My experience with the water based stuff is that it is not good for this application. It is a lot easier to work with but the price is quality of finish. I do a bit of polyurethane work around the house for example our kitchen table, a unique “bike table” in our dining room, two end tables, my big wooden desk, kitchen island etc. I have also done my grill slats many times, including with water based spar. I would say the water based might be OK for a surface that will be vertical but not for this. The grill slats are outdoors and laying flat. Therefore water sits on them and the sun hits them. The best polyurethane for this application is oil based spar. It is the only true heavy duty weatherproof solution I have found and I’ve tried them all. You do have to deal with cleanup but I have found that good technique allows me the ability to do the work with those foam pad brushes then I just throw them out. I like at least 5 coats and when I apply it, it is crucial to stir the poly (not shake) so you don’t have bubbles, then when I apply it I will paint it on and then when it is all wet do a final slow brush with a wet brush (foam pad). There are usually some very very small bubbles that pop as it dries. This final brushing is so slow, it’s like you barely are pulling the brush along, but not skipping it. All of the stuff I have done this way comes out great no bubbles smooth finish and durable.