I bought an RV-6 tail kit from someone and have started work on the HS spar. When I first looked at the spar stiffeners I saw what looked like brown sticky tape residue and dirt on the stiffeners. After cleaning and prepping with a scotchbrite pad I discovered that it was not tape residue it looked dark gray. The brown was just dirt and cardboard/paper fibers. It was corrosion. Most (80%) of the corrosion came off the surface, but some is still there after a significant amount of cleaning with a wheel. Do these parts need to be replaced?