Did you print inkjet?
Not sure about home remedies, but MicroMark sells a clear laquer spray fixative especially for inkjet decals. It;s possible any clear laquer my do. But TEST IT FIRST on a printed throw-away decal sheet of the same brand and the same ink.
For Alps decals, I never seal them on the sheet, just after application .
I would suggest using Microscale liquid decal film. You just brush it on and let it dry before using the decal. I have saved several old and fractured decals with this stuff.
I use Testors Decal spray made specifically for this purpose, which works very well. You have to be careful with Microscale liquid decal film on inkjet decals; it tends to smear them. You can also use artists clear fixative spray, available at art supply stores.