There is a can of oil-based polyurethane on most shop shelves, and there is a two-part polyurethane that a few cabinet shops spray for countertops and bar tops. They share a word on the label. They are not the same finish, and the gap between them is wide enough that one is a forgiving weekend product and the other can put you in a respirator for the rest of your career.
Start with the can you already own. Oil-based poly is an alkyd-urethane blend, and it cures the way an oil cures. It pulls oxygen out of the air and hardens slowly from the top down. Leave the lid off and it skins over. Brush it on too thick and the inside stays soft for a week. It forgives a damp day, a slow hand, a second coat you weren't sure about. The chemistry is patient because nothing is racing.
Two-part poly is a different animal wearing the same name. It cures because you mixed an activator into it, and the clock starts the second you do. The two parts are an isocyanate and a polyol, and they crosslink into a film that is harder and more chemical-resistant than anything the brush-on can will ever give you. That is the upgrade people chase. The catch is everything that comes with it.
Here is the part that bites on a humid afternoon. Formulators run a little extra isocyanate into the mix so every bit of the polyol gets used up. That leaves a small amount of isocyanate with nothing to bond to, and isocyanate will find water if there is any around. On a damp day it reacts with the moisture in the air instead of the film, throws off carbon dioxide as it goes, and you get bubbles in the surface and a coat that never fully hardens. This is why a wet day ruins a 2K film and does nothing to your brush-on poly. Roughly seventy percent humidity is a ceiling, not a suggestion. The same can that sprays glass on a dry morning will haze and pock in the afternoon if the weather turns.
Now the airway. The dry tone drops for this part, because it earns it.
The hazard with isocyanates is not the kind you can feel coming. A finisher can spray this stuff for years, under the exposure limit, with no symptoms at all, and decide it's fine. Then one heavy exposure, one bad day with the respirator off or the ventilation down, sensitizes you. After that, any trace at all can trigger an asthma attack. Not a strong dose. A trace. The smell from across the room. There is no working your way back from it, and there is no toughing it out. It is a step change, and the step is permanent.
So if you ever spray two-part poly, the gear is not optional and it is not the shop dust mask. It is supplied air or a powered respirator rated for isocyanates, full coverage, and the discipline to mix only what you'll use inside the pot life so you're not spraying free isocyanate any longer than you have to. You set that up before the first coat, not after the first headache.
The honest verdict is the calm one. The can on your shelf is the right tool for almost everything you will ever build. Tables, cabinets, doors, shelves, the brush-on poly handles all of it and forgives you while it does. Two-part poly is a real step up in durability and a real step down in forgiveness, in your finish and in your lungs, and most makers never need it. The thing that matters is knowing which one you're holding. They put the same word on both cans. The chemistry didn't read the label.