As Simon said, wood is porous. stain applied to raw timber will want to bleed a bit, or a lot... depending on how tight the grain is. some of the stains will also , if left long enough will stain plastic (as some of the guys dealing with binding can attest to. I f the stain if persistant enough to stain plastic thwn you may be able to seal the timber first , and rather than the timber being stained, the sealer takes up the stain, in which case masking may be more effective. The alternative is to seal the timber with a clear and then use tape and candy colours so you still see the grain.