I've purchased a few of the Lego keychain minifigs. Each time I've bought a new one was to replace the previous one because it had most or all of the paint rubbed off from going in and out of pockets and whatnot.
Here is my most recent example of the wear on my minifig keychain.
You can see that the mouth is almost completely missing.
Is there something that I can do to protect the paint and keep it from rubbing off?
This guide here (off-site backup) in the BrickJournal magazine has a section titled "Step Three: Protecting Your New Figure – Application of a clear top coat". It recommends Future’s floor wax or Badger overcoats and discourages the use of nail polish due to its tendency to yellow when exposed to sunlight.
They suggest applying two layers, each dried with a hair dryer to achieve a smooth, bubble-free surface.
Admittedly, these instructions are for display models, not figures that are going to endure the wear during play or everyday carrying along several keys, but these layers can be reapplied when they are damaged and will protect the figure and its paint while the overcoat is intact.
zovits recommends not using nail polish. I once had a chipped tile floor that I painted with nail polish. It stayed looking good for over 5 years. This was in a high-traffic part of my kitchen. The polish stayed shiny and prevented the tile from chipping further.
I suspect that a keychain probably isn't going to be sitting in the sun too much, but even so, the ABS plastic will discolour anyway, irrespective of the nail polish.
I would think a clear coat of paint would protect it fairly well, until the clear coat rubs off, so you may need to reapply periodically or apply multiple coats. Just make sure whatever you use is approved for plastics and it should work fine.
A thin coating should still allow most of the joints to work, if that isn't a requirement you could apply a thicker coating of a clear epoxy.
http://www.customminifig.co.uk/custom-lego-painting-guide/
Here's someone that paints LEGO's. The seal they use should work for you. You should apply more coats for more protection.
How about sealer?
Yes, always. If you spend this much time working on something, you want it to last. Sealer should be the last thing you do to your custom, painted parts before they are complete. I recommend an acrylic matte finish, but some people use gloss for certain things (it is supposed to protect better). I personally don’t want a “shiny” look on finished parts.
Another option is to use a gloss coat, allow to dry, then apply a flat coat for “maximum protection without the shine” (sounds like a commercial!), but I usually just stick with flat sealer only.
My solution to preserving your LEGO figures graphics would be to use translucent plasti-dip, which is normally used for tools to give them a grip as well as color code them. Assuming, of course, that you're using the figure as a key fob and not actually playing with it.
My first idea was that a clear, squeeze self-sealing coin pouch would work, but couldn't find one, then plasti-dip came to mind.
