If your printer has a PEI sheet bed (like the Original Prusa), printing PETG directly on it will eventually ruin it. PETG simply sticks too well to PEI. Or you might be having PLA adhesion problems and you want your objects to stick better. One solution is to apply a layer of stick glue on top of the bed. The glue will act as a barrier and will prevent the filament from sticking too much in case of PETG . You will still get great bed adhesion, but you won't destroy your PEI in the process.
Of course, if you're printing directly on glass, there's nothing to worry about as you can't damage it with any material.
In any case, printing on glue is fine and all, but trying to clean the glue afterwards isn't the most productive use of my time that I can think of. I've tried diluting it with water and wiping it with paper towels, but that's slow, messy and you end up wasting half the paper roll.
I've found that spraying a bit of window cleaner works the best. You don't have to drown your bed in it, just enough to cover the glue. You can use a small piece of paper towel or something similar to gently spread the cleaner around the surface.
Let the cleaner sit there for a minute or two. If there's a lot of glue on the bed, you might find that it quickly soaks up all the liquid. If you see that happening, spray some more cleaner on top. The end consistency should be similar to a pudding, not candle wax.
Next, find something long and relatively sharp, preferably made out of metal. I had some metal rulers lying around and they seem to be perfect for this.
Using a knife won't do as they're actually too sharp and you'll just end up scratching the surface. You could potentially use the spine of the knife, but then you'll have to flail around with a sharp edge pointed towards you; not the ideal or safest of solutions.
A lot of people are using metal spatulas for removing printed objects (and you should too!), which works just as well for removing glue.
Start at one end and slowly scrape off the glue. The glue should accumulate on top of the ruler or spatula, so you can actually remove quite a lot in one go. When there's too much of it, wipe or wash it away and then continue cleaning.
When you're all done, grab the window cleaner one last time and spray a small amount. Use a paper towel and give the bed one last wipe-down. You can tell if it's really clean by how shiny the surface is. If there are any glue spots left, that area won't be as glossy as the rest.
Your printing surface should now be ready for your next printing job.
Even though using a window cleaner makes it easier to remove the glue, it doesn't make it any more enjoyable. Consider saving your PETG prints until you bundle up a few of them and then print them one after another. You'll only have to clean the bed once at the end, which will save you a lot of time.