The International Cessna^® 170 Association

George you are correct and I agree regarding the JB Weld. I mispoke. What I mean (or should have said) was that JBW could be used to fill screw or bolt holes left in the firewall after some item had been removed from either the engine side or cabin side of the firewall such as an old power supply, etc. True, those passages where wiring, tach cables, etc pass through the firewall DO need to be a substance that is & will remain pliable, heat resistant and solvent resistent. JBW may not be good for the holes I mentioned, but it sure cures hard, but I don't know its compatability with the firewall material. Thanks for the correction.

I'm using hi-temp RTV, orange in color. We use it at work under the cowlings close to the hot section.Stands up well.

I see your point, Frank.
Another good method (one I prefer) to fill holes vacated by screws, etc....is simply install a screw, bolt, etc. Sometimes a large fender-type washer or countersunk washer held by an MS screw or bolt can fill old vacated holes, and I consider them much more attractive than fillers.