Paint

It's dead simple to apply.
You open the can and brush it on.
And it dries way faster than thinned epoxy.
(Some thinned epoxy NEVER cures or cures like rubber (if you use drugstore alcohol to thin it))

Where??????
On the firewall.
And inside the fuselage (behind the firewall) in the fueltank compartment and the areas immediately back from that tank compartment.

Why the firewall?
Because most ARFs aren't fuelproofed worth spit on their firewalls. There are often joints around the outside of the firewall that have poor coverage. The firewall often has an opening "through" it so the muffler can stay inside the cowl, and that's usually got incomplete fuelproofing along it's seams. Just because the front of the firewall looks like something has been painted on it for fuelproofing doesn't mean the something is adequate or has been adequately soaked into the seams. And if you make sure to paint in the hole through the firewall where the fuel lines come through, and you look to see your paint suck into that (often) raw plywood, or into that somewhat fuelproofed wood, you'll know why it's a good idea to follow up on the mfg's "fuelproofing".

Why inside the fuselage?
Someday the tank may leak.
For sure, the fuel line opening is going to allow oil/fuel into the fuselage if the firewall hole isn't sealed off. You're doing that with silicone seal, right?