We get asked frequently about how to store models safely, and it's a tricky question, since every storage location is different. Here are the basics:
Don't do it unless you have no choice.
First and foremost, I never recommend airtight storage for any length of time. Open-air display is generally recommended for plastic models whenever possible. Sealed containers like plastic totes hasten the aging process, as any off-gassing of the plastic is trapped within the plastic tote, much the same way you can ripen bananas more quickly by placing them in a closed paper bag. This is even more important for models made in the late 1980s - 1990s, which had a different plastic mix and are more prone to this aging process called vinegar syndrome. They will noticeably shrink and smell like vinegar if kept in storage, and eventually develop wet patches as the plastic begins to separate into gas, liquid and solid parts. This will cause fading of the paint and deformation of legs and bodies. It is not reversible. Below is a "shrinky" Breyer palomino Family Foal (right) next to a normal one. Her plastic has darkened, her legs are beginning to warp, and her body is starting to get lumpy-looking.
If models absolutely HAVE to be stored for longer than, say, a week for traveling, take precautions against heat, staining, and mold - All of which can ruin your beautiful horses.
The photo below is part of Triple Mountain's Model Storage facility - We have to store consigned models while they wait to be adopted, so they are stored in cardboard boxes on shelves with air spaces between the boards. The storage area is kept as close as possible to specs used by movie film archivists (Breyer models are made of similar material to old movie reels!) with a temp below 70F and humidity below 40%.
If you must:
When packing your models for storage, I recommend using cardboard boxes, which can allow at least a little bit of air circulation to reduce the speed of the aging process. Wrap models in soft white cloth, such as white t-shirts or white sheets between and around them, keeping them from bumping each other or the walls, top and floor of the box. Fabrics allow more air flow than bubble wrap, and air circulation protects them. (You can often find soft white sheets at thrift stores cheap, or ask around with friends and relatives, to see if anyone has any that they no longer need.)
Never use paper to wrap them, as it will take off their paint, and newspaper will leave ink marks on them. Colored fabric or threads can stain models, too, even after a pretty short time. (This is another reason to always untack models on display, too!) Tack and costumes should be placed in ziploc-style bags so they are not touching any model's plastic. Below is a Breyer Special Run Rugged Lark that was destroyed with red stains from being wrapped in clean, dry red fabric.
Packed models need to be kept below 76 degrees F (about 25C). Heat will cause legs to bend under the weight of the model (and models above them). I usually pack models standing up in the bottom layer of the box, with material around and between each. Then I'll wrap a layer of models to lay on top of them, and that's it! Two layers only, to limit the weight on their legs. I don't lay down the bottom ones because models lying down with weight on top of them is a recipe for bent or broken legs. Always choose the largest, strongest models for the bottom row, particularly standing horses with mainly vertical legs, which have the most strength. Roy the Belgian, yes... a Pacer, no. Models with fragile legs and tails, and those with bases, lie on top of the standers (like Rejoice, Show Jumper, etc.)
It's also very important to keep them safe from humidity/dampness. Any moisture can quickly lead to mold and mildew forming on the models, which can create permanent stains (and allergy issues). Below is what was a beautiful Breyer no-warpaint chestnut pinto Indian Pony that was stored in a basement that got damp. The same thing can happen in warm temps with high humidity, or if water gets into your storage area. Plastic models are like sponges - They soak up moisture, odors, etc. and provide a great medium for mold and mildew to grow on.
So, this brings us to the tricky part... Can you insure that no moisture will get to your models while they're in storage? If they're in a basement, attic, garage, or storage facility that doesn't provide humidity control, they're certainly going to get some moisture. We've even seen models stored in closets get mold damage after a flood or a roof leak. Obviously, cardboard will not protect well against moisture, and can itself begin to mold. So, even though optimal storage calls for humidity and temp controls and cardboard boxes, if your storage space isn't humidity controlled, you might be better using plastic totes. Be aware, though, that they are not completely moisture-tight, either! Whatever you choose to store them in, it's wise to buy a bag of silica packets and toss a few into each box. They can be purchased on Amazon and other retailers really cheaply, and they'll make a great first line of defense against moisture damage. The 5-gram packets below come in a 50-pack for $10.99 (as of the day I'm writing this). Clicking the image will take you to their page on Amazon:
They can only absorb so much moisture, though, so don't think they're magic. You still need to insure the models are stored in a dry location, and if they get exposed to moisture, you should replace the packets with fresh ones. Some can be dried out in an oven and reused - See manufacturer's instructions.
Regardless of your packing or storage methods, the most important thing is to check on your models monthly and after any big storms or high-humidity periods. Damage can come on quickly, so "forgetting" them in storage can be disastrous. It's heartbreaking to open boxes and find models covered in mold spots, stained or broken! If at all possible, keep them on open display. When they need to be stored, check them often... They're valuable and worth keeping safe.
[Disclaimer: Every model and situation is different. What works for most may not work for all, so please keep an eye on your models and adjust as necessary to protect them.]