Q:

Long term storage of agarose-ethidium bromide gels that have already been cast

I have come up with what I thought was a clever idea: Store the agarose gels I pour, and only cut as many lanes as I need to run later, minimizing wasted agarose (and wasted effort/time) when I need to make many electrophereses using a small number of lanes.
I realized that over time, the EtBr would diffuse out of my gel into the buffer that the gels are kept in. Since I add 2 µl of EtBr to 50 ml gel, and the gels were kept in 400 ml TAE buffer, I added 16 µl EtBr to the buffer, reasoning that now concentrations are equal and no net diffusion will take place.
However, after several weeks, I noticed that the EtBr signal from the gel is severely weakened. What can I do to fix this? I can see the following options:

Use higher concentration of EtBr in the buffer (how much?)
Change buffer weekly, adding fresh EtBr every time
Cover the transparent container with aluminum foil to prevent hypothetical photobleaching

Which one is most likely to solve my problem? Should I just give up, and throw away gels that are older than a day or two (I want to avoid this option, because of the record keeping needed to track when the gel was last made)?

A:

To increase storage life: after gel solidifies, dampen it with running buffer. Wrap the gel in polyvinyl chloride. Place in plastic container with a lid. Store in fridge in dark. It will last for a year as long as you re-dampen it with buffer each time you access it. Don't keep it submerged in buffer as the etbr will diffuse out. this is probably the cause of your problem. Notice when you order precast gels (either agarose or acrylamide) they are sealed and only a small vol of buffer is added to keep it hydrated. Additionally, the etbr and EDTA in the buffer helps prevent most mold growth although I have seen black mold grow a few times at around a year.
You can always spike the buffer with etbr adding it directly to the electrophoresis chamber before you run the gel (etbr doesn't even need to even be in the gel for it to work) although I find you get a little more sensitivity with the etbr in the gel. If you spike it be sure to mix and let it sit for about 20 min for the etbr to diffuse.
If you are trying to cut down on cost you are probably using a higher grade agarose than you really need for the application your doing so I would take a look at that
If you end up needing to store gels for rna work, add plain old Clorox bleach to a final concentration of 1% to your TAE gel just after microwaving. It sounds crazy but it works extremely well. You will completely inhibit RNAse and your rna bands will look nice!!!. The bleach also helps  with storage of the gel.

