The end grain of wood soaks up finishes and often turns much darker than the face grain. Check for this on your test block. If you get an ugly result, pretreat the end grain with a dose of finish that will limit absorption (wood conditioner, sanding sealer, shellac or polyurethane thinned 50 percent). Apply the treatment with an artist’s brush and be careful not to slop onto the face grain.
If you sand through the polyurethane and remove some stain, you can touch up with more stain. But the repair won’t be perfect, so take pains to avoid that mistake. Sand very lightly after the first coat, just enough to remove the dust whiskers. After the second coat, you can sand a little harder to flatten larger flaws. Always be careful around the edges of the table; that’s where it’s easiest to sand through.
After your table is pieced together, you will flip it over {remember, you've been working on it upside down} and sand it.  My husband isn't a big fan of the planks and worried about cups and plates getting stuck and spilled on the uneven parts of the boards, so he went to town with the sander. You can keep more of the plank look by sanding it less.

Building a table is one of the most deceptively simple woodworking projects going. What could be easier than gluing up a few boards and applying a finish, right? But reality is often surprising, because building a top for that table can go wrong in ways you might not realize until it’s too late.  Here are my favourite tabletop construction tips, plus advice on how to avoid trouble.