Build up to me would mean the build up of silica or other chemicals that lots of hair care companies talk about all the time, for which they recommend clarifying shampoos or shampoos without silicone derivatives etc.

We don't usually say "Care for buildup" as the French seems to be able to get away with, but "reduce" or "eliminate" are the most realistic we could use in this situation IMO.

"Silica causes many of the same effects on the hair as calcium. It causes hair to feel dry, lack volume, and can cause dandruff-like symptoms of flaking. Build-up of silica can choke the hair follicle causing hair to fall out."
http://en.wikipedia.org/wiki/Hair_care
"...Unfortunately, silicone-based hair products still get bad press in the hair-care world. They can build up on the hair and create a waterproof layer...A clarifying shampoo is needed to remove silicone buildup."
http://www.ehow.com/how_5813979_remove-silicone-products-hai...

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Note added at 18 hrs (2011-12-18 18:55:09 GMT)
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No nothing to do with the scalp. The residue or build up accumulates on the hair as the Silicone or other chemicals coat the outer surface of the hair shaft. This is what a clarifying shampoo is for.