Polyurethane finish question. I've used oil-based poly before; after three coats on a desk I built for my son, that finish is quite hard. It's difficult to scratch, to the point where I can't mark it with a fingernail. I feel like it cured to that level of durability in 3-4 days, but the toughness was evident 24 hours after the last coat was applied.
On a current desk I'm building for my wife, I grabbed water-based poly. The lower VOC count appealed to me, as the design mandates that I do at least the last two coats indoors (it's a built-in with trim work, so the big piece had to go in and the get finished out all together). I've got three coats on now, which is what the back of the can recommends. Applied with a foam brush, liberally. Sanded with 400 grit between coats as well, and all the application and drying time was at a temp above 65 degrees. The finish LOOKS amazing. Flat, no bubbles anywhere, I even managed to keep cat hair out of it. But it's not strong. I can scratch it with a fingernail. I know it will cure SOME over the next couple of weeks, but I'm disappointed right now.
Will it strengthen over time? Should I think about applying more coats? Am I up a creek? I just envision her using her craft supplies, or even just a laptop on this thing, sliding it around during normal work, and leaving light tracks. Those things add up over time, and I know it'll be all I see when I go in that room.