multiple layers of Def com ?

[rb1957]

We've got a situation where we have a gap (between structure and a door seal) which exceeds our guidance for thickness of Def Com (0.125"). The thought is to apply two layers of Def Com (after fully curing the first layer.

Thoughts ?
Adhesion between the layers ?? (rough up the surface ? apply 2nd layer with partially cured 1st layer ??)
Durability ? (we could replace later if service difficulties are anticipated ... we always have time to fix things later, but never enough time to do the right thing first ...)

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

RB1957... May be able to help... but unfamiliar with Def-Com...

Are you talking about DEVCON [aluminum, titanium, stainless steel, bronze, ceramic etc] filler/surfacer 'repair putty'... 2-part thixotropic epoxy paste with unique particulate-fillers for bulk strength or wear resistance?

http://www.itwadhesives.com/brands/devcon/metal-repair

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

yes ... DevCon

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Exactly which material [designated by metal filler type]?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

aluminium

another day in paradise, or is paradise one day closer ?

 

[moon161]

Without particular skill in holding planes together, I would ask why the guidance was 1/8 thick and whether adding a second layer was consistent with that thinking. With that answered, I would ask if the FAA would buy off on the idea, when audited, after inspection or after a failure.

 

[WKTaylor]

Rb1957...

As moon161 suggests using a liquid shim 1/4 thick is pretty sporty/chancy.

NOTEs.
A. 1/4" is the max-cure-thickness limit for one application of Aluminum Putty F [see link to TDS]... but building layers is practical, assuming each lay is mostly cured before follow-on layers are added.

http://files.engineering.com/getfil...ty_F~Aluminum_Filled_Epoxy_Paste_Adhesive.pdf

B. Surface prep and cleanliness is key to additional epoxy layer build-up. Old/hard-cured surfaces will need light grinding/sanding + wipe-clean [IPA dampened on process wipe] to expose fresh/clean cured epoxy.

I understand why a solid metal shim is inadvisable in this circumstance... if installed over a large area would become part of the structure... but thick liquid shim is probably prone to cracking and disbonding.

Recommend You consider fiberglass-epoxy shim: (a) pre-cured, trimmed/shaped to fit, primed, and installed wet with sealant with fasteners; or (b) pre-preg build-up [fabric or chopped-mat fiberglass], trimmed-to fit and cured-in-position.

Make from...
AMS3726/2 Shims, Filled Resin Compound Pre-catalyzed Sheet
AMS-S-83474 Shims, Molded, Filled Resin Compound and Sheet Prepreg
MIL-M-43248 [or equivalent] MATS, REINFORCING, GLASS FIBER + laminating epoxy resin [Your call]
SWAG: Boeing, Lockheed, AirBus, Textron, etc may also have pre-preg shim materials, also.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[TimMcK_Aero]

Late to the party, but anyway:

Epoxies are hygroscopic and can be brittle. Repeated thermal changes may induce some microcracking. Long term service may lead to corrosion problems due to a water pumping effect.

The metal filler may also scratch the base material.

You should also need to address the strain differences due to thermal effects with the larger amount of Devcon.

Late edit:
moisture get in Devcon crack
-> freezes (at altitude)
-> crack expands
-> thaws (landing)
-> more moisture.

 

[rb1957]

thx, I was looking for advice 'cause I didn't like the sound of the proposition. Fortunately this solution has been abandoned in lieu of a "proper" redesign.

another day in paradise, or is paradise one day closer ?