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Paint inside bolt holes

Paint inside bolt holes

Paint inside bolt holes

Hi guys,
I am new in this forum, but I often follow it and found very interesting information.
I have this situation on the Al. floor channels structure when the surface protection is not well defined and is not clear if is need to paint inside fasteners holes.
Both holes are drilled at H11, CSK on the bolt side.
Both Al channels are SAA+ Primer+Topcoat at detail stage, holes are drilled afterwards at final size at assembly.
Bolt Ti 6Al-4V, with SAA
Anchor Nut floating, CRES, passivated.
The bolt is wet installed with sealant.

The question is, after drilling the full size holes and expose the Al. substrate, what kind of protection is needed inside of the hole from corrosion point of view?
When using Primer+Topcoat, as there is no good control of the layer thickness, sometimes the hole is too small to insert the fastener. Personally I think only primer will be good enough.

RE: Paint inside bolt holes

Steve P... context for application/use of corrosion protective finishes is EVERYTHING.

A few specific questions...

1. WHAT aluminum alloy(s) and Tempers?

2. Do you have process standards and materials from 'brush application' of chromate conversion coatings [preferably 'no-rinse']

3. What primer are You using, IE: MIL-PRF-23377 BMS10-11, zinc-chromate, etc... or vendor PN equivalent?

4. What sealant are You using: MIL-PRF-81733, AMS-S-8802, AMS3265, etc... or vendor PN equivalent?

5. WHAT specific PN 'bolt'?

NOTE: Bolt Ti 6Al-4V, with SAA is incredibly confusing. For the Ti alloy, what is 'SAA' finish... doesn't represent any Ti finish that I'm aware of.

6. RE the CRES nutplate [=anchor nut].

What Specific PN Nutplate?

Is the CRES base truly bare-passivated or does it ALSO have a solid film lubricant finish?

What alloy rivets is it installed with?

What 'finish' [wet or dry] are you applying to the NP and NP rivets during installation?

7. What is the 'environmental zone definition' for that area of the floor channels?

NOTE. Every area [zone] of the acft should have an environmental definition. Typical zone definitions are...

Exterior exposed, interior dry, unsealed
Exterior exposed, interior dry, pressure sealed
Exterior exposed, interior bilge, unsealed
Exterior exposed, interior bilge, pressure sealed
Exterior exposed, interior integral tank
Exterior exposed, wheel-well or spar-well
Interior only, very dry
Interior only, bilge or condensation wet
Interior only, corrosive fluids exposure [toilet, battery, animal-cargo or other corrosive chemical exposure such as detergents/insecticides/disinfectants/etc]
Interior only, aircraft fluids wet [Skydrol, synthetic oil, fuel, coolants, alcohol, etc]

8. Experimental, commercial or military application? What is the expected service life after this work: Temporary, permanent + inspections, permanent/NO-inspections, 1000, 5000, 10000, 25000, +etc flying hours; and how many years = <1, >1 up-to-2, +5, +10, +25, +etc years?

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]

RE: Paint inside bolt holes

For once (and this may never happen again) I think Wil is overthinking it a little.
I'd take questions 2 & 7 only, and just do the alodine in most places (there are convenient products available).
Consider the implications of #7 if there are yucky things spilled on the floor, but even then, you should be protecting the floor as a whole, not just focus down on the individual fastener if you do have that problem. There is probably already a mat in those places, and furthermore if you have the floor ripped up, then a new one's going to be laid down once the structures work is done. The overall protection strategy will dictate how much additional protection is needed on these bolts.
I do agree with the wet sealant install. Are you using the product and fasteners recommended by the maintenance manual/SRM?

I am also unsure what you mean by the titanium bolt "with SAA". Assuming that is Sulfuric Acid Anodize, then either we need to talk about that, or it's just a typo.


RE: Paint inside bolt holes

Case of one distinguished fellow bit 'left' and & the other (brother SAIT) on the 'right'. As a veteran cabin & cargo floor repair engineer - i have always been annoyed at the lack of attention to fastener hole details. Vast majority of documentation will reference the typical conversion coating + primer aspect; but rarely detailing whether this information encompasses new fastener holes. As we all know floor rail corrosion initiates at radii, edges; nut-plate contact surfaces and especially nut-plate fastener holes. Worst combination is the CCR blind fastener in an untreated hole location. ... Guaranteed to provide hours of amusement for technicians & engineers. & this is not only an observation from MROs but even of new aircraft from the OEM! With the typical MRO time constains - bare holes are a "standard" now .. just watch.
.. related issue .. number of years ago partaking of a OEM structures working group - defining inspection areas & intervals; as long as the principle structural members were finished with conversion coating + primer - "no additional attention was required" - was the decree from senior engineers. Whereas early failure (corrosion) always originated from poor material selection & poor practices for the installation of nut-plates; inserts, secondary structures, ie: treating fastener holes.

RE: Paint inside bolt holes

Sorry to hear you're not a fan of "swamp rock", since you don't like "CCR"... wink
Didn't think I was being that provocative... so I could elaborate... or dig deeper...

The underfloor truly is a "swamp" in terms of being where all the bilge ends up and often doesn't drain away. If there's no protection offered at floor level to keep stuff out, then trouble is just bound to happen. It's more than just the infamous spilled Coke... the area is also the realm of all leak-able fluids, and the exhaust of the air conditioning system. Rarely is the exhaust air delivered in ducts all the way to the outflow valve, hence it just blows around through this big plenum. Given the frigid external temperatures at altitude, you can expect condensation. I've worked with some operators who started using a spray-on concoction of sealant and thinner and liberally applying it to the airframe in these areas, making them much more resistant to ocean environments.

The reality that the underfloor is usually kinda gross. If you think that application of alodine and wet-install of the fastener (esp. if that fulfills the OEM specification) is inadequate corrosion protection, but that additional measures will assure protection, then I believe you must check your definition of the problem.


RE: Paint inside bolt holes

we are talking the same thing - but at different levels. The just of my argument is that technicians are not 'trained' or 'informed' about the 'importance' of treating the insides of fastener holes or any adjacent 'bare surfaces' - just ensuring the majority of the part is treated & primed. In addition - the OEM manuals (as well as repair documents) never detail treating insides of fastener holes. Just imagine how labor intense it is to fab & finish the repair part with conversion coating. Drill the holes to final dia. & again finish the repair part (specifically fastener holes) with conv. coating + primer. In today's hectic workspace - these details are not promoted. .. if the AC returns with additional corrosion next time - all the better ! .. and who is to say previous work encouraged this additional damage.
Installing fasteners 'wet' with primer or sealant is obviously an improvement - but mostly used to seal pressurization or fluid escape; not typically used in general structural assembly. Installation of alum nut-plate spacers (between structure & nut-plate) is also another viable procedure - but rarely used. But my premier grievance is installation of CCR nut-plate fasteners as a 'time-saver' in an untreated hole .. a definite no no !

RE: Paint inside bolt holes

This is a cargo floor, could be as wet and dirty as it gets, scratches, oil, everting. It is not a pressure boundary on itself.

Anchor Nut: NAS1791C3-4 (CRES, passivated AMS2700, METHOD 1, CLASS 4, and Lubricated).
Screw: ASNA2001T3-xx (6AL-4V, SAA: Sulfuric-acid anodized, Lubricated).
Part 1: 7055 T76511 extrusion. My mistake, it is not SAA, but TSA (Tartaric Sulfuric Anodized)+ Primer+TopCoat).
Part 2: 7050 T451 plate, machined, TSA (Tartaric Sulfuric Anodized)+ Primer+TopCoat).
Not entirely sure on primer, but I think is an epoxy primer.
Wet install with PR1782.
Sorry, I don’t have info on the other questions.
I am also suspecting that the bolt maybe damage to protection inside the hole when inserted, or the screw head will stick out due the paint in the CSK. Using a larger hole might impact stress or fatigue.
I don’t have much experience with Floors, but for primary structure I don’t remember seeing TopCoat requirement inside of the fasteners holes.

RE: Paint inside bolt holes

Is it possible to choose materials that don't require coating in contact? and can the holes be 'masked' off using some kind of 'dummy' bolt if critical... also a little care in the application of the coating. Why fix problems if you can avoid them.


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