Fibronectin glass surface adsorption
What is the exact reason for the stickiness of Fibronectin?


Fibronectin interacts with diverse substrates which reflects its non-specificiy and its interaction with hydrophobic substrates is better suggesting that the adsorption is dependent on hydrophobic interactions.




  Does Fibronectin stick to the other proteins or rather to "free areas"
  of the surface?


It can stick to both as you can see from the table. However, its binding to hydrophobic materials like polystyrene and PVC is the most efficient. 


  which parameter is the best to vary?


Note that there will be a lot of free regions in the substrate. You cannot block all the sites unless you use a high concentration of fibronectin. So the best parameter to vary would be the concentration of fibronectin. You may also use fibronectin-BSA mixtures as their mutual binding is not that great (A stepwise method would be nonetheless better). You can also vary the adsorption time (see the figure below) but that is not as easily controllable as concentration. Since fibronectin does not bind to agar, you can also block some sites on your coverslip using molten agar (or agarose).





Reference:

Klebe, Robert J., Kevin L. Bentley, and Robert C. Schoen. "Adhesive substrates for fibronectin." Journal of cellular physiology 109.3 (1981): 481-488.