Aqua-Soft ForumsI own an iMac 17". A while ago my brother was doing some of this "novelty" mugs (where you spray a solution to the mug after you placed a personalized photo so it will stay. As he was reaching for something on the cabinet above my iMac, he accidentally touched the LCD with the photo paper he's holding with the said solution. Now my LCD has this blob on it and it's already hardened. Can you suggest something to remove it? He insists (since he doesn't know much of Macs) that it can be removed using alcohol but I said it might ruin the LCD. I already called the technical support to where I bought the unit and they weren't of any help at all. I'm really sad and frustrated on how a simple clumsiness ended up with this mishap.
iMac LCD trouble
Hmmm.... Interesting conundrum.
I will be honest and say I have never tried to clean anything like that off an LCD.
I do know however that you definetly want to avoid anything acetone based (Nail varnish removers etc.)
Isopropyl alcohol may do the trick as generally it does not effect plastics.
When I stripped the paint off my iBook I left the plastic casing to soak for hours in this and it was perfectly harmless.
However the plastic used on an LCD iMac is different.
The reason Tech Support wouldn't commit to an answer is if you ended up melting your screen they didn't want to be liable for any thing you may have attempted on their say so.
I suppose if its that bad I would probably attempt the Isopropyl Alcohol and if it did make it worse then claim off your home insurance (acidental damage). Call your insurance company an explain what your brother has done (don't mention the fact you tried to clean it)
Actually come to think about it...when I was cleaning the iBook up I used Isopropyl Alcohol around the edge of the screen to lift the dirt.... and in doing so I did get a fair amount on the screen... and that didn't mark it so it may be worth a try after all, after realising I had got it on the screen it was too late it had already dried so all I did was wipe it over again with a very very very lightly damp cloth and it came up very clean.
Its up to you mate bit of a catch 22. I would try it, but on the same note I would have been so angry that I would have beaten my brother to death with the iMac itself so at this point further damage would be irrelavant 
Will I use any kind of Isopropyl alcohol? Because some has 50% and some has 70%.
And about that iBook you cleaned, after it had some alcohol, did you dry it out with a dry cloth or did the alcohol just evaporated by itself?
I was using 100% medical grade Isopropyl Alcohol.
It evaporated so quickly there wasn't time to dry it off.
If your using a 50% or 70% just check what its buffered with (mixed), as I mentioned avoid any acetone.
If your going to try it I would put some on a small cotton bud and only if possible apply it to the part with the adhesive, might take some time but its probably a bit safer than covering the whole screen.
The only thing is... depending on what solvents are in the adhesive mark on the screen they may have already damaged the plastic by etching it already 
Use nail paint remover.... just take care with the drops...
@ capeli: Haven't u been reading?..One big ingredient in nail polish remover is acetone..acetone dissolves plastic..so that's a no-no. I doubt however that isopropyl alcohol will do the trick, as alcohol doesn't dissolve the kind of paint kaloyster discribes. The stuff used to protect the picture on the mug is probably some kind of clear paint, that stuff is rugged. Now bear with me now, cuz this might get a little complicated, but I promise I'll keep it short. Most of the time, the same kind of ingredients that keep paints or other similar stuff in liquid form, are the ingredients to use to get them liquid again, after they have hardened. The good news: If you can just find out the ingredients of that solution ur brother used, u might find the right type of solvent to make it liquid again, so it can be wiped off carefully! You want to avoid smudges, or it will mess up ur display even more. Ok, here is the bad news. If the plastic of ur display and the solution have somehow bonded together, chances are u won't be able to remove all of it. So do it at own risk! If all of the above instructions makes you too nervous, just use brute force: scrape the solution off with a razorblade. May the force be with you..
Use nail paint remover.... just take care with the drops...
WTF?!
did you not read the other posts!?
Have you seen what the majority of Nail Polish removers do to plastic?
I while ago I managed to spill a few drops of the stuff on my Apple Keyboard.... and I am not exaggerating when I say I moved like lightning to get the stuff off.
Where those drops fell I now have a matt and near frosted finish on my keyboard.. thats the keys and the clear back plate.
I would strongly advise not using Nail Polish (paint) remover or any thinners for that matter.
Edit:- Sorry vale just beat me to it