After having done some additional tests, bonding epoxy to the phenolic firewall
material as well as testing of my previous firewall bonds, I have satisfied
myself
with a conclusion. It is that epoxy does not form even a somewhat adequate
bond to any unabraded glossy phenolic firewall material. The bond is almost
non existant compared to that of an otherwise epoxy to unabraded epoxy bond.
So leaving any glossy phenolic surface unabraded is well more detrimental
to the overall bond than leaving some unabraded area under a epoxy to epoxy lay
up.
I still consider that on the areas where only one ply of glass/epoxy is over the
foam sandwich, one has to be careful not to sand off too much of that single
ply to get down to a complete abraded surface, not so on the phenolic firewall
material.
The front (engine side) of the firewall, requires less sanding and material
removal
to get below the glossy surface, as it is smooth having been against the
mould in the original lay up, compared to the less smooth firewall backside with
its depressions from the embedded woven material. So you just have to remove
more material on the firewall backside to get below all the gloss for an
adequate
bond.
Contamination on surface was not the culprit in my case. Though removal of real
or potential surface contamination is alway a necessary step in bonding
preparation.
Bill McClellan
-----Original Message-----
>From: William McClellan <wilwood@earthlink.net>
>Sent: Apr 25, 2010 11:30 PM
>To: "Europa-List@matronics.com" <Europa-List@matronics.com>
>Subject: Europa-List: Re: epoxy to firewall bond
>
>
>Thanks for the replys, I am at a bit of a loss as I followed my usual routine,
scuff sanding (80 grit)cleaning with alcohol then allowing to fully evaporate
(clean paper towel), laying up the 4 ply on cellophane to insure proper wetting
of the layers of glass, and I had done the surface prep just prior to layup
so not likely and air borne contaminates got on the surface. In this case I
left the cellophane on top as I used the instrument panel against it to
establish
the mating of the flange and the panel. No peel ply used. I would guess
that if any mold release was used in the original creation of the fire wall, it
was on the other side, in any case as usual, I took the surface down to newly
exposed material. I have tried to separate or pry off an epoxy to epoxy
surfaces
before and it is undoable, but has to be sanded or ground off. I have also
had to remove a reduxed phenolic pad and it also can only be ground off.
Maybe it only leaves to do another test lay up as aga
!
> in see if I can pry it off.
>Thanks,
>Bill McClellan
>
>
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