Something I thought might be good to mention in case others have similar problems.
I received a call about 6 months ago from a guy I painted for (a little over a year ago) during his build. I finished painting, showed him the mural and said good-bye (it was about 2 AM). He didn't have black lights of his own yet, but when he finally bought some and put them in his soffit, he fired them up, anxious to see the mural again. He left them on for an hour (10-15 minutes is all that's needed), turned them off and... nothing. He saw only a few stars. HE tried again, leaving the lights on all day. Nothing.
He called and told me that the mural just wasn't charging. We discussed things that he might want to try out to see if he could get the murals to charge up. None of them worked. I was at a loss. I told him that when I was on the east coast again I would run out to his house and see what I could figure out... and if it just wasn't charging, I'd repaint his ceiling (large ceiling).
I got there last night at about 11 PM and there were a lot of lights, from his equipment, lit up... which we turned off. He'd had his 10-12, 48" black lights on all day charging the paint... and I was anxious to see what was up with the mural. Well, we turned out all of the lights and.. nothing. I saw about 20 stars, out of maybe 5000 stars. I couldn't understand why the paint wasn't charging... even the paint that was about 12" above the lights.
Then it hit me.... and the point of this post.
Usually the tube black lights come with a clear plastic shield (can't think of the name right now) to protect the light from getting hit and breaking. In normal situations the cover is left on, but I have found that most of those covers will block the light that charges the paint. It lets some of the light out, but it seems to block most of it. The light that I carry with me when I travel has the cover removed for that very purpose.
OK, so I asked him if the covers were still on the light, which they were... so I got up into the soffit, removed the cover from one light and then we did a test. We turned on the black lights for about two minutes. But, even during those two minutes we could see a difference in the paint on the ceiling. It was fluorescing around that light, and no others. When the lights were turned off... around that light that had it's cover removed... the paint was charged us as it should have been.
We removed the rest of the covers, turned the black lights back on for a few more minutes, turned them off and the mural was glowing as it did the night it was painted.
Relief set it to both of us.
So, if you have black lights that still have the plastic cover over the lights... try taking them off and see if the mural charges up any differently.
If you are about to install black lights... be sure to take the covers off.
I should have remembered that, when giving him options to try... but after taking a couple of hour drive out to his house, I doubt I will forget again if I get another call about the paint not charging.