How can I increase adhesion on a hardly filed nail?

As long as you remove the shine from the nail, you are in good shape. Excessive filing on the nail is not necessary. Use a 240-grit file in the direction of nail growth until no oil or contaminants are left on the surface of the nail. A 240-grit file is gentle and shouldn’t cause the client any discomfort. Most acrylic lines make a product or combination of products that will cleanse and dehydrate the nail plate; a process that should leave the nail chalky white. Use a primer and make sure your product consistency is what it should be according to manufacturer’s recommendations, since different products call for different ratios. -- Wood-Lundy