Can I put Rogaine on top of scalp that already has hair building fibers on it, such as Toppik?

You should apply Rogaine to your scalp (not on top of Toppik). The medication has to reach your scalp.

It is best to apply topical minoxidil to your clean scalp before using any other products.  If you use the foam (which dries faster), apply it to the scalp and rub it in before using any concealing products.  

Yes, Rogaine should be applied to a clean scalp. Toppik fibers and other similar keratin fibers will reduce absorption. 

From personal experience, you don't WANT to do this!  It gets all clumped and does not let either product work as effectively.  Try it once - you will see what I mean...Good luck!

When you use Rogaine, you should have the hair and scalp clean of any impediment that will stop it from being absorbed such a Toppik. Take off the Toppik before using Rogaine for the best resulst from this drug