27 - Firewall - THE PRIMER WARS!

To Prime or Not to Prime?

It is accepted knowledge that the stainless firewall should not be primed - the FAA requires, for certified aircraft, that the firewall be able to withstand 2000F for a period of time. This means that if the firewall was primed, the paint would just burn off, creating potentially hazardous fumes. So the stainless will remain un-primed. 

However, what about the aluminium parts attached to the back of the firewall? 

The issue with not priming these parts is corrosion - there are a number of stacked parts here, meaning lots of places water could penetrate. In addition, the parts are aluminium which are touching the stainless firewall - meaning the possibility of galvanic corrosion. 

On the opposite side, is there a risk of these parts off-gassing fumes if there was a fire? 

I asked Andy, from Stewart Systems, his thoughts and had this reply:

We have not done lab testing on it, but I can tell you it will not be any issue unless your engine compartment is on fire.  I have painted Continental cylinders with Ekopoxy and Ekocrylic and it has help up very well.  I also did spark plugs with that combination.  They discolored a little, but otherwise have held up great over many hours of operation.

Thanks!

Andy Humphrey  A&P/IA
Tech Support

So based on this, the paint should hold up perfectly to the normal operating temperatures in service. The only issue would be if there was a fire. 

So - the decision. I decided that the risk of a fire was fairly low, however the risk of corrosion was very real. So all the aluminium parts attached to the firewall will be cleaned, etched and primed with the normal EkoPoxy i have been using for the rest of the aircraft.  

Firewall Sealant

The plans call for the use of fuel tank sealant along the seams between the firewall sheets - to stop any CO2 or noxious gasses during a fire from entering the cockpit. There is a lengthy thread on VAF around the use of fuel tank sealant, and the result of some real world testing is that it is not good for use on the firewall. As such i have purchased a couple of tubes of 3M Firebarrier 2000+ for use on my firewall.