Today I sealed the forward bottom skin and forward side skins to the firewall, and got them clecoed and ready to rivet as soon as I find a bucking partner.
I started out by dimpling the firewall flanges (with substructure dies), then cleaned the primer off the bottom angle across the inside of the firewall and down the inside of the forward edges of the side skins to make sure the sealant would adhere properly. I also scuffed and cleaned the bare stainless firewall flanges. I used FlameMaster CS1900 in a SEM kit to do the sealing. This stuff sets up way faster than tank sealant and it was already getting stiff before I got the parts clecoed in, but I made it and it squeezed out nicely. I clecoed every single hole around the bottom and sides of the firewal, then clecoed the rest of the side skins and bottom forward skin normally.
Firewall flanges dimpled.
I used clecoes about half way back the floor stiffeners to hold the bottom skin off to apply sealant.