Citristrip is a paint stripper. It does not truly etch the metal cups, merely removes the paint or powder coating. Think of it as a reverse stencil - rather than painting, the citristrip removes the paint in your stencil area.
This is the product I used
As for the cups I used, I just pulled a variety from my stash. It worked beautifully on the Tal mug, and did not work at all on the contigo mug. The good news is, if it doesn't' work, it doesn't harm the cup at all - I can still apply a vinyl decal the to Contigo Cup.
The contigo mug AFTER washing off the citristrip. Although it did not work on this cup, it also did not damage the cup at all.
I don't think this is a clear issue by brand. It will vary by paint finish, so if one brand of cup comes in two different paint finishes, one may work and the other may not. It's going to take a lot of trial and error - but in general, look for a more matte finish, or a powder coat. The shinier finish seems to be less likely to peel.
Method
Apply your stencil - make sure it's VERY secure! I ruined one mug already because my vinyl was not applied tightly enough. You can use stencil vinyl, or, as I did, just use permanent vinyl. It's a really good idea to take a hair dryer over the vinyl once its on the cup - you want it very firmly stuck fast, and the hair dryer will soften the adhesive and make it stick better.
Give yourself plenty of buffer around your stencil area. Add painters tape if you need to. The citristrip may run a little, you do not want it to run down and touch exposed areas of your cup.
I used a foam craft brush to apply a thin later over the stencil.
Let it sit for at least an hour.
Use a diaper wipe to wipe off the citristrip. Now take a clean diaper wipe and wipe a bit harder - the paint should be coming off. Keep wiping, scrubbing a bit, until you have most of the paint off.
Remove your stencil, wipe some more.
Then I rinse mine off in the sink.
Not all of my edges are super crisp, but overall, I'm really happy with this technique!
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