Does anyone have any ideas on how to repair and protect cracked/peeling decals, like these pictured in my 10019 set:
Some kind of film or lacquer?
Oh, those look sad! Stickers with white designs on them are notorious for peeling/cracking.
I have found that the best way to protect stickers like these is to apply a thin layer of clearcoat to them while they are still new. Wait until the clearcoat dries, then apply the sticker to the LEGO set. You can use pretty much any acrylic clearcoat, medium or varnish available at hobby and craft stores. Anything in the scrap-booking/decoupage section should be safe, but it is a good idea to test them on a small corner of the sticker-sheet. Always apply the clearcoat thin. You only need a thin layer over the sticker. If you apply too much the clearcoat can yellow with age.
Once stickers are already damaged it would be pretty much impossible to repair them. Even if you try to put a clearcoat on an already damaged sticker, it will continue peeling, so it is not worth it. It is much easier to just get and extra sticker-sheet for any LEGO set on Bricklink. The stickers for your set above are here.
I have actually made it a habit to always pick up 1-3 extra sticker-sheets on Bricklink when I buy a new set. While the set is still new and readily available the sticker-sheets are much cheaper (often just a few pennies) than trying to get a pristine stickers-sheet for a set that is 10 years old. Just place them in a ziplock bag, and store them away from heat and direct and sunlight. They won't deteriorate that way.
Replacement stickers from a third-party site are an option, but they can be expensive - the sticker sheet for this set is currently between $75 and $200!
You can repair printed paper stickers, if you want to take the time and care to do it. The basic steps are:
Remove the sticker from the piece. I use a razor blade. Hold it nearly flat against the brick and very carefully start around the edges of the sticker until you have enough pulled away that you can hold an edge with your fingers. Then it's a matter of pulling evenly at the sticker while separating it with the razor blade. This takes patience. Go slow.
Once the sticker is removed, flatten out the wrinkles and re-align the torn spots as much as you can. I usually do this on a piece of waxed paper. Any glue left over on the sticker will hold on the waxed paper, but it can easily be peeled off. Once you have it flat and smooth again, put another piece of paper over it, then a piece of cardboard, then some heavy books or even a brick. Leave it under weight for a couple days.
Clean the sticker residue off the brick. You might be able to rub most of it off, but I have also used products like Goo Gone (which is petroleum-based, so use very sparingly). After getting the residue off, wash the brick with warm soapy water.
Take your sticker out from under the weight. You can do some cleaning of the sticker, if needed, using a kneadable eraser. If the sticker is mostly intact - that is if the layers of paper haven't peeled apart - you can now carefully remove it from the waxed paper and reapply it. In some cases, there will be enough glue on the back that you can just re-apply it to the piece as is. In other cases (like the example above, for instance) glue is missing or the layers of paper have peeled apart.
In the case of peeled layers, you can re-glue them together using a thin layer of glue - like from a glue stick. Carefully apply it to the top of the lower layer of paper, position the printed paper smoothly over it and put it back under the weight to dry. If part of the backing paper has torn away (and is left stuck on the piece, for example) you can replace it with very thin tissue paper.
If the layers are intact, you can restore the stickiness using "repositionable" glue. This is glue that you apply and let dry for a minute before putting the sticker back on the piece.