Attach 100-grit sandpaper to a hand sanding block. Sand the surface of the poplar using strokes parallel to the grain. Repeat sanding the poplar with 180-grit sandpaper until the surface of the wood feels smooth and silky to the touch.
Wipe the dust off the poplar with a soft cloth. Repeat wiping the surface of the poplar using a tack cloth until the wood is completely free of any dust or debris.
Put on safety glasses and breathing protection. Dip a staining sponge into an open can of prestain wood conditioner. Apply a wet coat of wood conditioner to the surface of the poplar using the sponge. Wait 15 minutes for the conditioner to dry.
Open a can of Varathane oil stain. Use a flat stick to thoroughly stir the contents of the can. Dip a clean staining sponge into the stain. Apply a wet coat of stain to the surface of the poplar with strokes parallel to the grain of the wood. Apply the stain to all four edges if they are exposed. Wipe the stain off immediately using a soft cloth. Wait two hours. Check the color and apply an additional coat of stain onto the poplar for a darker color.
Apply your choice of topcoat after waiting 24 hours for the stain to dry.