Firewall to skins: Fay seal now or fillet seal later?

[Draker]

The instructions only mention this once, and say only "Seal all [firewall] openings with fuel tank sealant after skinning the fuselage." No guidance on whether to fully fay seal by sandwiching a layer of sealant between the skins and firewall, or whether to apply a fillet seal to the junction after the surfaces are mated.

Van's builder support recommendation is to not do it while installing the side skins, but to do it later and "run a bead around the inside."

Far be it from me to question the all-knowing mothership, but the number of builders who chose to fay seal their firewalls gives me pause. Are these builders making a mistake, is there a trade-off here or is this something that's truly builder-preference?

I understand that the firewall does not get riveted with the skin install due to the cowling hinge that goes on later.

Also, I'm not trying to start a debate on which sealant to use. I've read the superb "Firewall sealant and Fire Safety" thread. Only looking for pros and cons of sealing it between the surfaces vs. putting a bead around the firewall later when everything is already mated.

[PilotjohnS]

I added sealant when I put the side skin on. I cant see how one would get the sealant on the seam near the engine mount brackets if it was done later. JMHO

[Robin8er]

I also added sealant when I put the skin on. I think thats the best way.

[jcarne]

I put a bead of this stuff (per Dan Horton testing) between the side skins and bottom skin and firewall. When the top skin goes on I will put a bead on the engine side then fireproof the firewall and put another bead. If you haven't looked up Dan Horton's firewall testing threads definitely do.