Why is pine difficult to stain?
Michael Dresdner: It’s really quite easy to stain. The problem is making it stain evenly.
In pine, the earlywood and latewood are very different. The earlywood is soft and spongy, and absorbs lots of stain. The latewood is harder and absorbs less stain. This often results in a photo-negative effect when the wood is stained. To make matters worse, the wood contains random pockets of sap. If you use a stain that contains the solvent for that sap (any oil-based stain), the sappy areas will preferentially absorb extra stain, creating patches of darker, uneven color.
If you are staining pine with oil-based stain, do yourself a favor and first treat the wood with wood conditioner. Flood it on, wipe it off, and stain immediately while the conditioner is still wet. Or, seal the wood first with a THIN coat of Zinsser SealCoat. Flood it on, wipe it off, and let it dry completely before staining. Different techniques and materials, but both will result in a more uniform, albeit lighter, stain.