I was refinishing my pine floors. I sanded the floors down to bare wood with a drum sander. I used a high quality tinted wood varnish. However, after completing the second coat I noticed that dark patches were forming where one brush stroke meets the next. The dark patched only seemed to form in these areas where the brush strokes ended. I pressed with the third coat thinking that the patches will start to blend in, the patches did decrease slightly but are still evident. I'm unsure of where the problem is? I felt like I did everything right. So the questions are what could have caused this problem? Is it the problem or my application? And how do I remedy this situation?