Interesting defect

[FigureLLC]

this is something i haven't seen before and is blowing my mind. i'm coating an aircraft control panel made of aluminum. there are rivets holding mounting brackets to the back of the panel. my customer wanted the rivets, which were already very flat to the panel, filled/smoothed. i said no problem, they're shallow enough that a primer coat should take care of them. so i hogged on some KL primer so that no rivet was visible. when i baked, the powder seemed to actually flow away from the recesses instead of into them. so i wet sanded them out using 100/220/340 grits wrapped around a flat wood block, down to metal in spots. this time i used powder-to-liquid to apply dabs of primer on the rivets only. when baked, this left raised bumps of primer (so i *knew* i got those suckers this time). i flatted them down then applied the grey powder. upon baking, i could still see the rivets - clearly - and something odd had happened... anywhere there was bare aluminum, the finish was dull (this won't matter bc its final coating will be a liquid urethane). but i thought it odd. this time i used a DA at low speed with 120 to again flat the rivets. i finished off the whole panel with 220 wet on a block. i *did not* go down to bare metal this time. cleaned it up and applied grey again. baked again and son of a bitch, it looked almost exactly the same with the rivets visible and the dull spots.

1) what would have been the correct way to deal with these rivets from the beginning?
2) what can i do about them now?
3) why the heck is the primed part glossy and the bare metal part dull (i could understand the first time maybe, but after i laid a second coat on a first coat that was sanded uniformly?!)?

you can see how drastic the difference in this pic.

[FigureLLC]

47 views and no suggestions?

[CarWiz]

LOL... I looked at it twice and can't figure it out. About the only thing I can think of is that there's oil or some fluid behind the rivets that's been there since assembly and it's weeping out. Did you give it good bake before coating? As far as the flat/shinny area, that could be temperature variations. The open area will heat faster than the coated area and may be over-cured. But, this is just a guess too. (Or it could be part of the contamination too.)

Ken

[SCOTTRODS]

this is something i haven't seen before and is blowing my mind. i'm coating an aircraft control panel made of aluminum. there are rivets holding mounting brackets to the back of the panel. my customer wanted the rivets, which were already very flat to the panel, filled/smoothed. i said no problem, they're shallow enough that a primer coat should take care of them. so i hogged on some KL primer so that no rivet was visible. when i baked, the powder seemed to actually flow away from the recesses instead of into them. so i wet sanded them out using 100/220/340 grits wrapped around a flat wood block, down to metal in spots. this time i used powder-to-liquid to apply dabs of primer on the rivets only. when baked, this left raised bumps of primer (so i *knew* i got those suckers this time). i flatted them down then applied the grey powder. upon baking, i could still see the rivets - clearly - and something odd had happened... anywhere there was bare aluminum, the finish was dull (this won't matter bc its final coating will be a liquid urethane). but i thought it odd. this time i used a DA at low speed with 120 to again flat the rivets. i finished off the whole panel with 220 wet on a block. i *did not* go down to bare metal this time. cleaned it up and applied grey again. baked again and son of a bitch, it looked almost exactly the same with the rivets visible and the dull spots.

1) what would have been the correct way to deal with these rivets from the beginning?
2) what can i do about them now?
3) why the heck is the primed part glossy and the bare metal part dull (i could understand the first time maybe, but after i laid a second coat on a first coat that was sanded uniformly?!)?

you can see how drastic the difference in this pic.

I'm pretty certain there's a plating on the rivets,.... cad plate or something like that. One can only speculate on how you got this odd effect...... Could be from a difference in chemistry of the metal/s and the coating on the rivets,.... Contamination from the rivets from original assembly (some sort of lube or oil or graphite, maybe even paint).

I'd recommend giving the piece a bath,.... in some 1012. (or MC). After stripping completely, Outgas for a good long time. Then apply the primer.

It is kinda wierd to see things like that. We could all learn that valuable lesson of what not to do if you get a solid complete finish with no issues.

Let us know what you do and how it turns out please. This one's a keeper.

[FigureLLC]

i ended up just block sanding it, and i appear to have gotten the grooves around the rivets out, and i sent it off to the printer. i'd have preferred to strip it and start from scratch, but deadlines...

i'll ask the customer what kind of rivets he used. they were ground off flush, and sanded by me, so i don't think it was contamination of any sort. i ask what kind of aluminum sheet this was, too. they're aircraft control panels, so maybe they're a weird alloy.

i did not try prebaking these pieces since they're just aluminum sheet. mighta coulda helped.

so we may never know what the heck happened on these. when i get some time, i'll shoot some samples and try to duplicate this thing.

[SCOTTRODS]

i ended up just block sanding it, and i appear to have gotten the grooves around the rivets out, and i sent it off to the printer. i'd have preferred to strip it and start from scratch, but deadlines...

i'll ask the customer what kind of rivets he used. they were ground off flush, and sanded by me, so i don't think it was contamination of any sort. i ask what kind of aluminum sheet this was, too. they're aircraft control panels, so maybe they're a weird alloy.

i did not try prebaking these pieces since they're just aluminum sheet. mighta coulda helped.

so we may never know what the heck happened on these. when i get some time, i'll shoot some samples and try to duplicate this thing.

Len, I wouldn't have thunk it either, as it is sheetmetal, but the cooking for the rivet joints to clean out,.... that's new to me too, but makes sense now.... Kinda like outgassing, but more burning out oils or greases. The sheet was likely formed and joined with the rivets before any of the process coatings were removed. In most sheet metals, there's almost a "need" to put something on the metals, to allow them to be stacked and not stick together. I don't think they "oil" aluminum, but they do all steal products I have warehoused. It wouldn't be out of character for the handlers and manufacturers to use something as a "release agent" for the stacked sheets. ....

Anyway,... We'll be watching to learn of there's any further info gained from your experiences with this, including the "stealth camo" thing.

[FigureLLC]

i might be implementing a pre-bake-everything policy.

i'll try the reproduction attempt in a camo pattern!

[SCOTTRODS]

i might be implementing a pre-bake-everything policy.

i'll try the reproduction attempt in a camo pattern!

Sounds great too. After reading the "stealth Camo" comment, I've been contemplating doing a black on black on black camo utillizing flat, gloss, and texture blacks. It just piques my imagination.

[welterracer]

From my experince i prebake everything.. It sucks but it works..

What you have in those pictures looks like oil or something oozing out from behind the rivets and getting into or under the powder as it flowed out.

Ive had that issue several times with grease..

[rsmarco]

tell me more about this camo thing you are speaking of....