What exactly causes ghosting?

[CES020]

I'm well aware that being exposed to light fades materials and you end up with ghosting, but I've had a few occasions that made me wonder about it a little more. I've had various materials, most recently anodized aluminum (clear anodized) and I put some vinyl lettering on it, customer called in a change, so I removed it less than 24 hours later, all still sitting here in the shop. I peeled the vinyl off, no issues, used adhesive remover (not really any adhesive left on it, but used it anyway), and then rapid tac to clean that off. However, when the light hit it right, you could still see a faint ghost.

Was that much damage done to the surface in 24 hours, indoors? Or is it something more than fading, it's a reaction to the glue on the surface, or what?

I'm just curious this morning. I've also noticed on something like black acrylic, you can do the same thing and it only shows up when you mist it with a cleaner.

Any ideas what it is that's actually causing the ghost? Light? Adhesive etching into the material?

And for clarification once again, I'm not talking about long term vinyl projects exposed to light for long periods, I'm talking about very short term and still seeing the ghosting.

 

[Just Another Sign Guy]

i've had a couple of occasions where immediate ghosting was noticeable one was on a painted aluminum sign and one was on brushed aluminum both had to have something changed after no more than 48 hours after installation.

i've always assumed that it was uv exposure and in both of these instances about 2 weeks after the change was made you could not tell where the vinyl was that was removed. so again i really think it was uv exposure.

 

[Gino]

Like Dan said.... ghosting is primarily something that happens from the sun, but it can also happen from minute dirt particles in the air and other contaminants that we breathe in. When road dirt and other things land on the sign, the vinyl lettering protects the area where it is. After a buildup of that on many surfaces... along with with the sun U/V ..... oxidation will form. When the vinyl is removed you will see a slightly different hue, but mostly notice how shiny it is there.

I believe what you're experiencing is some sort of static charge or change in the surface tension. When something is glued fast.... you've actually changes the molecular structure of that surface and it takes a while to come back. Looking sideways at it or lightly misting the area will show these results, but again Like Dan said.... it goes away eventually.

So, this is not truly ghosting as we know it in the business, but just a slight temporary change in your substrate's surface.

 

[AUTO-FX]

I am wondering, isnt that just a process of oxidation? I think everything we use in our business is subject to oxidation of various degrees, aluminum being near the top of the list (although you would assume anodized aluminum should have some level of enhanced protection). Direct uv exposure isnt needed. I too have seen it and also noticed it gone later on. Maybe when you peeled off your letters, you removed the oxidation leaving a fresh surface ?