Naptha to thin polyurethane. Brush strokes should not be noticeable if you use a good brush like a Wooster if you use self leveling polyurethanes. I coat hardwood floors for a living with the stuff.
If you're using waterborne poly then a foam brush will give you good results. Apply it and don't play with it. Just use strokes that run back into the wet product and keep a "wet edge" . You need to work fast but not furious. Naptha is great if used with a mineral based product. It does make a great thinner for spraying. Check your product.
