I'm a bit surprised no-one has picked up on your question, but a shellac finish would work well. It forms a film and is quite water resistant. You can apply it with a brush, rag or spray. You could even do a traditional French polishing job. All these methods need some practice to get a good result because, for instance, if you put it on with a brush you'll find you can't really go back over polish you applied even just a couple of minutes before without making the appearance worse. So with a brush, it's a case of get it on quickly and not too thickly and leave it alone to dry. Then, when dry rub down lightly what you've applied with something like 400 grit paper and apply another coat. Repeat until you get a satisfactory film, usually two or three coats will do it. You can wax it later if you want and this will dull the sheen, but this choice is optional and not really needed.
Over time, poplar (or tulipwood) will change colour from its natural slightly muddy white (often with green or pink mineral streaks) to a sort of honey brown, and those pink or green streaks turn a dark brown. Slainte.
