Try the below way. Calculate the volume of each coating and multiply it by density of the coating. Then, the outcome will be weight.
The surface area that FBE coating(Outer surface) * the DFT of the FBE coating (Max DFT recommended) * the density of the FBE coating + The surface area that ARO coating applied (Outer surface) * the DFT of the ARO coating (Max DFT recommended) * the density of the ARO coating
Note that the above way is kind of conservative by considering "outer surface area" & Max DFT.