Paint on threaded fasteners

[RHTPE]

Some interesting questions for those who are very familiar with the performance of threaded fasteners:

- Given a 1/2-13 bolt & nut:
[ul]
[li]How does a coat of bare metal primer affect the nut-to-bolt connection? Good, bad or otherwise?[/li]
[li]The threadlocker folks say the paint would adversely affect the ability of the threadlock compound to bond to the metal - agree or disagree?[/li]
[li]If this were a vibration-prone connection, would the presence of paint on the threads adversely affect the long-term security of the connection?[/li]
[/ul]
This question was posed to me and quite frankly I had to admit that I don't have a clue.


Ralph
Structures Consulting
Northeast USA

 

[TehMightyEngineer]

Here is my educated guess, though I've never tried this with a structural bolt and wouldn't normally accept a bolt that wasn't cleaned properly:

-I would think the paint would have no affect structurally for the most part. Shear is unaffected and unless there was so much paint that it couldn't be threaded without damage, then I suspect tension would also be unaffected. Getting a proper pretension might be an issue as the paint would compress like a spring and thus not give a proper pretensioning. This concern would be my primary objection to this. I also would expect much difficulty threading the nut onto the bolt and the potential for cross-threading/wear on the thread during the bolting. You might also not get the nut tight against the connected part as the nut will likely chew up a lot of paint into a big pile behind the nut which might end up holding the nut off of the connected part.

-I think it's a "maybe" on the threadlocker. I imagine most of the paint will be worn off during bolting and the threadlocker would bond to the now bare steel-to-steel surfaces. Where it's bonded to the primer I imagine you would have reduced performance but not eliminated. Depends on what you're using the threadlocker for. If it's vibration related then I imagine the paint would jam up the nut similar to a locking nut. However the pretension issue would be a concern for vibration and I would consider this more of a concern.

-See above. I suspect it would either result in something acceptable as the paint will "gum up" the threads. However, there's so many "maybes"
here that this seems like way too much trouble than it's worth. If you have to go with this then I would make some sort of mock-up and test the results for vibration and pretension. I'm assuming you're priming before bolting so that then you can walk the whole assembly into a paint booth and paint? I'd think a more efficient method would be to just bolt with plain bolts and hit them with a spray primer and then paint normally.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[RHTPE]

TME - I agree with the many "maybes". From what I understand about the situation, the paint coating is not thick enough to adversely affect engaging the nut on the bolt. It was just a thin coat of primer.

I would not have condoned painting before assembly.

What I suggested was a test using a handful of fasteners - half painted and half not. Tighten to a prescribed initial torque. 48 hours later examine the torque required to unthread the nuts. All done with threadlocker of course.


Ralph
Structures Consulting
Northeast USA

 

[IRstuff]

Seems to me that the paint/threadlocker question depends somewhat on what's in the primer, specifically. Since you're effectively changing the chemical composition of the threadlocker, various things could happen, from strengthening the bond to weakening the bond. Hypothetically, primer is an adherence promoter, so it should behave more like a threadlocker than not.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[TehMightyEngineer]

Ah, so this is something already built that needs to be justified.

I'd also vibrate the piss out of the test articles. Stick them in a paint shaker machine or attach a concrete external vibrator to them. If you get little change between them then I'd say you're probably fine with the caveat that this testing the waters.

How much trouble is it to go back and replace the bolts/nuts?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

It somewhat depends on the type of paint that was used as the primer. If acrylic, it will flake under the stress of thread engagement. If oil based or solvent based, it will smear or gall. If a threadlock is placed on the painted surface, it will either reject the paint or meld with it. If it rejects, then it won't perform. If it melds, it will likely perform but have a delayed reaction. Give it time to "re-cure".