we all need alittle more info, but since not supplied. you need to remove the grease, film, and grime so that any additive will not bead up or skip that area because it cannot penetrate.
I would use any sort of remover to clean off the grime. Then I would use a fine grit sand paper and sand in the direction of the grain, then move to 220 grit.
Then I would put on a good sand sealer, then again lightly sand due to grain rise. Dust off well then apply your stain. Stain with a rag and wipe in the direction of the grain.
Then apply your finish coat, either water or oil based polyurethane or verathane. If exterior door, use a good UV protectant.
This is the very basic of the job.
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