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Is it safe to use automotive polyester resin fillers on aircraft structural aluminum?

Posted By:
Donald Weigt
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#1 Posted: 4/11/2010 01:53:25

A 1984 Stits catalog lists MP-1100 MICRO-PUTTY polyester resin filler material.  It also says bare aluminum surfaces should be prepared by scuffing with a Scotch Brite pad and treating with E-2300 Chromic Acid Conversion.  Is this really a problem?

I've tried using automotive light weight polyester resin body filler around some gussets to smooth the transitions to the tubes they are riveted onto.  I broke some off after a few days, and the aluminum beneath was shiny bright.  It sure didn't look corroded!  These are on the tail parts of a Challenger Light Sport Special.  The gussets and tubes are aluminum, the rivets are stainless steel or aluminum.

The filler is only meant to smooth the surface under the fabric covering.

Is it safe to use automotive grade polyester resin based body filler on aircraft structural aluminum?

 



Joanne Palmer
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#2 Posted: 4/11/2010 09:44:27

It depends is the real answer.  The manufacturer should state on the can if it is OK to use on aluminum; and with aluminum being used on some cars, it should be OK most of the time.  The automotive fillers tnd to be somewhat heavy tho... 



Joe Norris
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#3 Posted: 4/11/2010 12:42:20

I'd suggest using something like Alpha Poxy and microballoons for the application you describe. Very light and very flexible.

 The acid etch is to remove any oxidation, oils, or other contaminants so that your epoxy will adhere properly.  If you don't do the acid etch, you are likely to have adhesion problems.

Cheers!

Joe



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Andrew Ovans
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#4 Posted: 4/13/2010 00:45:45

I will second the expoy instead of polyester.



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Christopher Carlson
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#5 Posted: 4/13/2010 23:18:37
Donald Weigt wrote:

 

A 1984 Stits catalog lists MP-1100 MICRO-PUTTY polyester resin filler material.  It also says bare aluminum surfaces should be prepared by scuffing with a Scotch Brite pad and treating with E-2300 Chromic Acid Conversion.  Is this really a problem?

I've tried using automotive light weight polyester resin body filler around some gussets to smooth the transitions to the tubes they are riveted onto.  I broke some off after a few days, and the aluminum beneath was shiny bright.  It sure didn't look corroded!  These are on the tail parts of a Challenger Light Sport Special.  The gussets and tubes are aluminum, the rivets are stainless steel or aluminum.

The filler is only meant to smooth the surface under the fabric covering.

Is it safe to use automotive grade polyester resin based body filler on aircraft structural aluminum?

 

Donald;

You may fine the best avenue to "level" the contact area of tube-in-tube and tube-to-angle joints is a simple bulidup of felt strip.

On Quicksilvers and Flightstars we used to have problems with uneven tube joints rubbing thru the fabric...a layer of felt (or Duct Tape...I live in the South after all...) was used to bring the smaller tube diameter up to the larger tube dia. A topper strip was added if the joint was in or near a corner.

 

                            Hope this helps.

                                                           Chris

 

 

 



Tom Mitchell
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#6 Posted: 4/14/2010 21:23:32

I have to agree with Chris.  Polyester based filler on aluminum is likely to separate from aluminum because of radically different rates of expansion/contraction during temperature changes.  Epoxy should be better, but felt would be best - and lighter.  You could use a spray adhesive, like 3M 77, to attach it to the transition areas.

Tom



Donald Weigt
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#7 Posted: 4/14/2010 23:46:00

I found the manufacturer lists aluminum as an acceptable "substrate", so I think it'd not be the source of a corrosion problem or some other major headache.

That said, I've just spent two days carefully removing it from the tail parts, except where I plugged the ends of some tubes with balsa blocks and used the filler to smooth the surface.  I was concerned about the bond strength and aluminum's large expansion and contraction with temperature extremes, too.  I didn't want to end up with loose bits of resin stuck to the covering but not to the airframe.  I'll use some variant of the felt idea.  The manual says to use tape to cover the rivet heads and sharp edges.  I'll try the felt, and also just adding a few layers of tape.  The gussets are only 0.025 thick, so maybe it won't take too many.

Thanks, everybody, for your comments and assistance!

Don



Tony Pileggi
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#8 Posted: 4/15/2010 16:41:29

As long as you prep it properly, there should be no problem. Vinylester would be better. Epoxy is no better than Vinylester, but costs a lot more and is much harder to use.

 

I wouldn't use any of the above for structural applications without the proper tech data on preparation of the aluminum, and then I would rather use mechanical fasteners.



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