At least once a week I am asked questions about gluing images. The most commonly asked is how to prevent bubbling. Bubbles are caused when the glue dries too quickly in spots creating a pocket of air and does not adhere to the surface. This is especially a problem when working with larger images. Applying the glue (thinned with water) with a brush helps somewhat but you must be ready with the brayer. Sometimes I will apply glue to sections, but this has its own set of problems. One sure way to help the glue absorb into the paper is by wetting the paper first. This slightly "breaks down" the fibers in the paper to allow the glue to absorb, BUT, the paper becomes a little more fragile and must be handled carefully.
There are a couple of ways to rid your masterpiece of bubbles. The first is by lancing the bubble with an exacto knife and reapplying glue. this works but you must use a steady hand and try not to tear the paper. Make the incision just large enough to be able to slide a flat object beneath the paper (a flat toothpick works really well). Using you brayer flatten until the "seam" is not so visible. A note about brayers: keep the surface of the brayer clean. A brayer caked with glue can attract grainy particles which can tear or mark the paper.
The second technique is using a syringe with a large needle. The needle needs to be large enough so that glue can pass through. First with the plunger pushed in, puncture the bubble and draw the air out by pulling the plunger back. I am not sure if plunger is the correct term, but this will flatten the bubble. Next load the syringe with water thinned glue and squirt into the bubble using the brayer to smooth. This works very well.
Never use too much glue. This will cause the paper to tear and will clog the brayer with glue sometimes lifting a sizeable area of the image. Too much will also not allow the paper to lay flat and it will become difficult to work with. Apply the glue evenly and quickly if using a large image. foam throw-away brushes are good for the job.
Keeping your hands clean is also important. I keep a wet cloth or paper towel and a bowl of fresh water nearby and constantly wipe my hands. Not only will your fingers stick to the paper and cause tears but you are also more likely to pick up dirt.
Scrap cardboard (I call them glue boards) is a perfect work surface for gluing. The larger the piece the better. By using large pieces you can rotate the cardboard to ensure the surface is free of wet glue, and once you are through toss it.
When applying the paper, start in the center and work towards the edges using a damp cloth. This takes a lot of work. Push out the excess glue. Once you are satisfied that you have done your best, seal the edges. Using your thumbnail or a popsicle stick rub over the edges firmly but not enough to tear. Pay special attention to the edges making sure they do not lift.
All of these techniques take a little practice but any problems can usually be solved. If you have a tear you can repair or cover with another cutout. But if your paper does not adhere then sealing is useless.