There are many who, when finished with their Diamond Painting, seal it. The reason for this is that you want to make sure that no drills are going to fall off from it. Whether or not you really need to seal it people have different opinions on. Some say that if you frame the painting behind glass then you do not have to seal it. Because then the glass will keep the drills in place. Others say the drills can fall off anyway. That the glue on the fabric gets bad over time and makes the drills come off from it.
If you want to seal your Diamond Painting then you will find some information about what is usually used below. I have personally not yet tested any of them, but I will soon enough.
Many people use Mod Podge to seal their finished works. Mod Podge is a decoupage medium. It can be used as both a glue and a sealant. There are many different varieties. Those used mainly for Diamond Painting are gloss or sparkly. Some people think that after having used Mod Podge, the painting has become less glittering or has become white. So it can be a good idea to test on a less successful painting before trying on the one you are most satisfied with. I will write a more detailed post on how to do with Mod Podge after I tried the product.
There are also glue that can be used to seal Diamond Paintings. I have read that many have used a spray glue and gotten good results.
I have also heard that it is possible to use Clear Elmer Glue.