Need to use solvent or water based pre clean... That gets any grease off the part and don't touch it anymore after you clean it. Use two rags, one soaked with the pre clean, the other dry to wipe off RIGHT after wiping with the pre clean.
76GMC1500, your comment about the silicone reminded me of something... last year or semester shall I say in my welding class at the local college, the teacher told the class about welding his friend had to do for the government and military on vehicles, chairs, etc. He said that they were VERY picky and things needed to be done PERFECT with no problems and done on time. Well they were TIG welding something and got done and the weld was as perfect as it could get. The inspector came by his shop and looked at the part. He said the part was very good but he cannot take it. He said there were very little black dots the size of a pinhole on the bead (just a few of them). They could not figure it out until one day my teacher went to an old guy and showed him that... The old man asked him "what are you doing to clean these?" He said sand blasting... The guy tells him "well if your sand blasting then you have silicone in the blasting media causing this... That was the problem was the silocone... lol sorry I am off topic but thought I'd share that with you.. Maybe I actually taught someone something.