Let's face it. No one looks at a bottle of Mod Podge and feels the need to read the instructions. Myself included. And then we wonder why we can't get the results we're looking for! So here I'm going to add some tips and tricks for how I've perfected my own technique using Mod Podge to avoid bubbles, warps, tackiness, etc. I hope they help!
First, prep your surface. This isn't totally necessary, but I've found it makes me feel like everything goes together nicer. Simply rough up your surface lightly with a quick once over of sandpaper. No need to spend any serious time doing this, but 3 or 4 swipes isn't going to make this take any longer than it already would. Especially on smooth plastics or glass.
Using a flat paintbrush or a foam brush (I personally prefer the paintbrush because of the control I get and the ease of cleanup), paint a thin layer of MP onto your prepped surface. Next, paint a thin layer onto the back of the paper you are using to cover your project - by the time you have this layer painted on (do the edges last - they are key to keeping everything smooth!), your layer on your project should be dry to the touch - if not, you probably used too much.Carefully adhere the paper in place on your project and immediately smooth any bubbles or creases with your fingers. KEEP smoothing for a minute or two. This will keep bubbles from forming - use your fingers, use a bone folder, use a brayer, whatever will help make sure the two layers of MP bond together smoothly as they are drying.
At this point, I generally do my trimming or sanding to remove the excess on the edges. My favorite tool for this is those cheap black fingernail files. Once you're done with this, make sure you wipe off ALL traces of filing dust etc.Apply a thin coat of MP over the top of your paper and allow to dry completely. Even though MP dries quickly, for this step you really need to be sure you're in a fairly non-humid area and allowing a really thorough dry in between coats. A fan can help too. I'm generally working on several projects at any given time so I'm not as concerned with the time it takes to let one of them dry but if you are in a hurry, it really is worth the extra time to allow about an hour between coats. Building layer upon layer of not quite dry MP is what causes half of your stickiness problems - the other half are caused by humidity! Add as many layers to the top as you'd like - I usually use 3 just for good measure.If you are adding more layers of paper or stickers, they should be added after the first top coat, following the above instructions as if the new addition is your original paper but giving it more drying time as in the final steps. This will allow your project to build up gradually around your extra layers, smoothing everything out without making it sticky.
If you are adding a sticker, first remove the stickiness! Seriously - stick it onto some fabric a bunch of times, use Un-du, or dust it with cornstarch or baby powder. Then use the Mod Podge as your adhesive. For dimensional stickers that you want on top, wait until all your layers are dry and then add that sticker last. I usually add a wet adhesive (E6000 is my favorite) just for good measure. I use this for flowers, metal, and any other embellishments I want to not move ever again. It's goopy, it smells bad, and it WORKS.
First, I recommend using the Mod Podge for Paper formula - this one is acid free (and comes in matte or glossy), unlike the original. Also, a big part of how Mod Podge works is that it is meant to kind of soak through the paper. Obviously, this makes photo paper challenging. You will have MUCH better results if you either color copy the photo or print it onto good quality printer paper or thinner cardstock.
Using Rubber Stamps: Mod Podge is a liquid so if your ink doesn't say "waterproof", it's going to smear if you don't protect it somehow. You can use clear embossing powder and heat emboss the image or you can use some type of a spray fixative (I like Krylon's matte finish) and wait for it to dry before Mod Podging over the top of it.
Another thing if your project is something like a mini book where the pages will touch - use a spray fixative (matte or glossy) over the final coat of Mod Podge. Dry thoroughly (at least overnight if you can't give it a few days) before closing the book. Helps eliminate any stickiness that might be left.