Well, like I've said before, rats learn by doing what other rats do. They go right up to other rats noses and mouths to get at what they may have. Whether that be food or something to chew.
First, it may suck if you have a small colony, but you can narrow down who the chewers are by simply getting rid of the ones in the tub that was chewed out of. This stops any would be chewers from teaching other rats.
Some rats have malocclusion problems. This literally means their teeth grow sideways or outwards. It makes it difficult for the rats to eat, but they may have found a way to keep the teeth to a good size by chewing the tub. If they didn't, the teeth would grow up and outwards, often through their skull. It is genetic and passed from parents to offspring easily.
Just something to keep in mind. I remember reading about someone that had rats with weird teeth that would chew through flat surfaces. He had to get rid of the entire group and start over. Since then, no chewers.
Look at the design of your rack itself. Does the lip of the tub meet the wood framing perfectly? Is there any gap at all? When you make the slider frames, try for the tightest fit possible. Over time the gap will most likely increase with use, simply from rubbing on the hardware cloth screen. It will get easier to open them.
When we first made my rack, it was a PITA to open, especially with one hand, and rats in the other. Over time it has worked into a very easy opening tub.
Making it tight as possible will limit the rats from getting a tooth around it.
What are you offering for feed? Lab blocks are hard and help wear down the rats teeth, taking care of a lot.
Do you offer any wood blocks to chew? You can usually find untreated wood in 2x2" or 2x4" at home depot for a good price. I cut down a few sticks of that into 4 inch sections and give the rats a couple new pieces every few weeks. Some rats just pee on them, but many use them. Especially pregnant or nursing females. They seem to go through them like crazy.
Lastly, are you giving the adult rats enough room? Tightly, confined rats are going to be stressed and rats deal with stress by chewing.
Rats love scraps. Have any scraps of chicken bones? Leftover hard foods? Give it to the rats.
Hope this helps a little, that's all I could think of off the top of my head.
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~C
