: Hi Jeff,

: The published strengths are when using bare aluminum, the trick is
: getting the aluminum bare (which isn't really hard, just dirty)
: and keeping it that way till it's glued.

: I've painted and glued aluminum and sometimes it works and
: sometimes when it's done exactly the same way it fails. No real
: reason why other than the oxides form quicker it seems in some
: alloys and/or possibly weather issues for how the oxides form.

: The aircraft industry has a published method that works, but is too
: expensive for home use and it's licensed anyway. A guy named
: Molt Taylor came up with a method for homebuilt aircaft of
: sanding the aluminum while in a bath of diluted epoxy that seems
: to work for home built aircraft, the epoxy bath keeps the oxides
: from forming while they are being sanded away.

: Bill H.

Bill, I've heard other people report exactly the same experience you had. The other technique I have read about, besides sanding in epoxy, is to 'passivate' the aluminum by treating with phosphoric acid. Presumably that removes the oxide and forms an aluminum phosphate surface.

Loctite does not specify how the aluminum was pretreated in their tests, but they do say that the only treatment required before gluing is a solvent wipe. West Systems, on the other hand, does say exactly which pretreatment gives which test results. If Loctite did pretreat the aluminum to remove oxides before gluing and testing then their technical data sheet is deceptive, but that should not surprise any of us too much.

None of this really matters of course, since I have no idea what bond strength is required.

Thanks as always for your comments,
Jeff