How come you're going to use painted poplar? To save money or because you really want it to be white? How long do you plan on living in this house? I'd say if you're going to put your blood, sweat, & tears into something you're going to keep for a long time and be proud of then you ought to use some nice material like walnut or cherry, but not if you're going to paint it white, and not if you plan on moving soon.