Agreed.The water in the polyurethane raises the grain. It could have been prevented by sealing the wood first with Zinsser Sealcoat however all you have to do is sand the finish smooth and apply more coats. Each time it's sanded between coats the finish will get smoother. I would recommend using 220 grit or finer paper for this.
You are referring to an oil based polyurethane where the topic was a water based polyurethane. Anything you put on wood is going to raise the grain, even an oil based polyurethane. Mixing mineral spirits to oil based polyurethane won't make it any less grain raising. It will just take more coats to achieve the desired finish. Personally I believe a person should thin an oil based polyurethane as little as possible. Applying it by hand I don't thin it at all. I only thin it for spray purposes.I was told to mix mineral spirits in equal parts to poly and it will not cause the rough finish. Is this true?