Disclosed is method of preparing microspheres wherein the microspheres do not increase the self-extinguishing time of a composition they are added to by more than 5% and their presence does not increase a viscosity of the composition by more than 65%. The microspheres are coated with a very high level of from 80 to 90% by weight of a flame retardant, preferably aluminum trihydroxide. Unexpectedly, the presence of the flame retardant on the microspheres at this high level reduces the need to add additional flame retardant to a composition along with the microspheres. Also unexpectedly, the location of the flame retardant on the microspheres completely prevents the normally expected increase in viscosity of the composition based on the level of the flame retardant added. The microspheres find use in many compositions requiring flame resistance including weld through sealants and adhesives.