I have noticed that poplar, if left for some time before application of stain, will apparently take on a deeper color due to oxidation or some such effect.  If the poplar is not completely sanded over, the stained surface will show lighter parts where the most recent sanding has occurred.  

I have found it very difficult to get glue marks out of the poplar.  What is the preferred way to get these out before stain is applied?

What is the preferred way to stain or color poplar? I have read much on the subject of staining woods prone to blotching; the most effective method seems to be two coats of 1 lb. shellac, followed by a coat of tinted shellac.