OK, as promised, here are the steps I take to place an image seamlessly onto a crown cap like these for a blonde ale I have yet to brew...
-Golden Matte Medium
-Heavyweight Presentation Paper (Matte Finish)
-Light Colored Bottle Caps (Silver, Gold, White, Yellow, etc)
*At least for ink-jet images, the ink is transparent, so it's advisable to stick to lighter colored caps to better see the image.
I first start by designing and printing an arrangement of images smaller than 1x1 inch square in photo or best quality mode. The bolder the image, the better. Also remember to print the image mirrored since it will be inverted onto the cap.
I use the acrylic matte medium pictured below to paint a thin layer on the blank cap...
This 8oz bottle will probably last me a lifetime...ha.
Center and press the square on the cap. I use a burnisher to evenly press the image across the cap surface...
After about 5 minutes, you can place the cap under running water and begin to lightly rub the paper off...
The reason the image stays on the cap and doesn't wash away is why it is important to use the kind of paper I did and not regular ol' copy paper (I tried). There must be a coating that protects the acrylic from re-emulsifying under the running water. When most of the paper is removed, you may need to examine it briefly under a light to make sure there is no more paper pulp left over...any left on the cap will poke through the lacquer that goes on next.
Here you can see a before and after lacquering...
You might be able to see a few bits of acrylic build-up along the edges...they should be carefully scraped off with a thumbnail or something before lacquering for the same reasons as the bits of paper pulp.
I just use a spray lacquer from Home Depot and apply one heavy coat from all sorts of angles around the cap(s), being careful not to let it run. In 20 minutes it is dry to the touch.
I assume the lacquer will help it to hold up to submersion in water, sanitizing with something like Starsan solution, and regular handling. I wouldn't try to boil them... a practice I'm not a big fan of anyway.
That's about all there is to it. It may seem like quite a bit of work just for a cap, but eventually, you get the system down and do sort of an assembly line procedure... the time per cap gets greatly reduced. I had a ton more pics to include, but there seems to be a limit of 4 images, even if they are hosted somewhere else...oh well. I'd be happy to answer any specific question anyone might have that might not have been answered in this post. Good luck and have fun!