If you have kids, you know how their favorite books is destined to fall apart...
There are some DIY-techniques to repair a book, the most well-known is probably clear tape. that is often good choice, but it doesn't work for all applications.
Vinyl repair is a good alternative when tape doesn't work, both for gluing papers together and applying on places which will bend, fold, and stretch a lot, as the spine of the book. Here I will use the excellent Performix VLP (Vinyl Leather Repair), which I consider to be an extremely useful and versatile type of glue/ fill. It will fill gaps, stay flexible and stick to almost any surface.
Step 1: Clean the Surface If Needed
For best result, clean with denaturated alcohol. Dry, non-gloss paper does not need cleaning.
Step 2: Adjust the Edges
Adjust the edges of the teared paper so they are as close as possible to each other. Even if vinyl repair will fill gaps very good, joining the teared edges will make the appearance as good as possible.
VLP actually dries fast enough to keep the edges joined with your hands until it dries, (drying time is similar to a layer of nail polish). The best is to fixate with tape though, a layer of food plastic wrap can be used to cover the wet glue, in order to fixate with tape. After drying, the tape and plastic wrap is easily removed.
Most of the time though, adjusting and fixating the edges is not a problem.
Step 3: Apply the Vinyl Repair
Next, apply the vinyl repair. It doesn't matter if the layer is thicker in some places, it will dry and form a thin film anyway.
As always, read and follow the safety instructions on the package!
Step 4: Q&A
Q) What is the difference between vinyl repair and paper glue?
A) Vinyl repair will fill gaps, creating a thin, flexible plastic bridge covering the gap. Think of it like liquid tape, but without the re-attachable sticking feature. It will also glue paper sheets together, like regular paper glue.
Q) How long time time before it dries?
A) It will be dry to touch within minutes, but I assume full strength will need hours. I think it behaves much like flexible nail polish, and it smells like it, too.
Q) What appearance will the glue have when dried?
A) It will be a thin, clear, flexible film, glossy like high-gloss paper.
Q) Will it really stick to a high-gloss book cover?
A) Not very good, unfortunately. I don't know of anything that does, the best I know of is tape but it is not fit for all jobs. VLP will peel off of high-gloss surface eventually, but it will stick very good where fibrous paper is exposed. The pieces that are peeled off can then easily be removed by tearing them off, leaving only a tough, flexible film on top of the exposed paper. For best results, the glossy layer on the book cover can be removed (peeled off) but this will not look very nice.