This is kind of an odd question, but how do you prevent tires from "drying"? After leaving the cars on the shelf for a while, the tires get some sort of layer on them and loose a lot of the grip. It can be rubbed off, but not completely. What's wierd is it doesn't happen to all of my cars--bits don't, for example, but digi-q's, epochs, and bit clones do. Its a much larger problem for the smaller cars.
Any way to fix this?

Sandpaper?

try soaking them in hotish warm water (this happened or something like it 2 my xmod soft tread and slicks) for a few minutes

Is the "coating" white? Like powder?

No, its clear, but if you look at the tires in the light it seems just the slightest bit glossy.
I'll try the hot water thing, hope it works.

lol i forgot 2 explain why this would work
well it works becouse rubber can lose moisture just like skin and when ruber and skin get dry enough they loose thier gripiness and when u have realy hard dry hands you soak them in hotish water and it makes them soft again(my step dad does this couse his hands are hard and full of blisters 2 romove them[the blisters])
... idea u could probaly use potroleum jelly but then u gotta wipe of the jelly
(the water maybe the jelly is good for xmod and smaller size tires any bigger might not have much effect

watter isn't a good moisturizer for rubber products. It will soften them temporarily, but as soon as the water evaporates, the rubber becomes hard again. The smaller cars it's worse on because the thin rubber tires become dried out all the way through the tire, not just the surface. Some rubbers are more prone to it than others. especially soft rubbers. Take a rubber band, leave it outside in the sun for a week and then go back and take a look at it. Its hard and cracks when it's stretched. Its impossible to reverse this effect. If you have a thick tire and only the surface is effected, you can run it on some fine grade sand paper, just touch it to the wheel as its spinning and sand the surface layer off. IF the dry-rot goes all the way through, then you need new tires. Things that advance dry-rot in rubber are Sun exposure, and dry arid conditions. If you have a sealed display case your tires will last a good long time. Especially if its made from U.V. shielded glass!

Also, A quick tip that works well for mini-z/epoch tires is

Soak your tires in a film cannister (or similar) of WD-40, check them every hour, once they seem a little softer (maybe 2 hours), take them out and spray them with "clean green" - this takes away the oil from the WD40 and leaves the tires nice and tacky

Ah, the perils of living in Southern California...
Hey, the WD-40 just might do the trick, it's worth a shot especially since replacement digi-q tires are impossible to find.

heck, they already screwed up, might as well try something, it is not like you can ruin them any more....right?