The Murphy Builders Site

-----Original Message-----
From: tjpackard@mmm.com [mailto:tjpackard@mmm.com]
Sent: Wednesday, January 24, 2001 3:53 PM
To: Murphy Rebel Builders List
Subject: Re: PR-148, PS-890-A-2 and preparation



I'd like to introduce myself (Tom Packard) to the group and
comment on the
tank sealing issues that have been discussed recently. I'm a first time
builder, Super Rebel 043 and have put about 1800 hours into it over the
last three years and I'm halfway thru the fuselage construction.
So far so
good, and like others, the wet tanks were the worst part of the project so
far. When I pressure tested my tanks (@ 2 psi), they held pressure for 7
days with only minor pressure fluctuations up and down with temp. changes.
When I built them, I did a few things differently as a result of my
experience at work. I work for 3M Company's Aerospace Lab/Adhesives
Division and from my experience with adhesion to aluminum after
ScotchBrite
scrubbing, blowing the surface off after the ScotchBrite prep is not
sufficient to obtain good adhesion. A layer of ScotchBrite and Aluminum
particles will remain on the surface and reduce adhesion at the
ProSeal/Alum interface. It is true that many towels/rags may
have starches
or other contaminants that could be a problem. We use unsized/lint free
cheesecloth and 91% Isopropyl Alcohol or other solvents to wipe
the surface
clean before applying adhesive material to aluminum. Other solvents are
more effective, but are more toxic than IPA. eg. MEK, Toluene, etc.
Another thing I did to make sure I got a good seal was to rivet a
length of
stringer across and near the top of each of the two center tank ribs so
that the top flange was straight and would line up with the holes in the
top skin when I brought the two together. This prevented having to move
the rib flange around (and the ProSeal) for hole alignment. I
believe this
helped to keep enough sealant between the flange and skin to get a good
seal. I also made different corner brackets and stringer seals on both
sides of the outboard tank rib. I'm keeping my fingers crossed but I'm
optimistic about the results.

Tom P

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