Foam based fluoride solution has the following advantage EXCEPT-
Foam based agents were developed in an attempt to minimize the risk of fluoride over dosage as well as to maintain the efficacy of topical fluoride treatment. The advantages of foam - based APF agents are :
1. It is much lighter than a conventional gel and therefore only a small amount of the agent is needed for topical application.  
2. The surfactant in the foaming agent has a cleansing action by lowering the surface tension. This also  may  facilitate the penetration  of  the  material  into interproximal surfaces where its action is most needed. 
3. Since  APF  foam  does  not  require suctioning, it offers advantages for home use as well as for the treatment of young children and  disabled  persons where saliva evacuation may not be feasible.
Minimize the risk of fluoride over dosage
It is much lighter than a conventional gel
The surfactant in the foaming agent has a cleansing action by increasing the surface tension
Used in patients where saliva evacuation may not be feasible.