I'm not telling that stripe vendor anything. Besides, I see no good reason to spend money on a stencil kit for straight stripes.
You might have installed the stencils before the paint had "gassed out". Failure would then not be the kit, but the user. I had this problem with stripes on a GMC Motorhome that the customer was in a hurry; we laid them on too soon against our better judgement, and it took two more days to strip and repaint. 3M technical folks told us about the paint gassing time being too short.
Glenn Uettwiller
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"I'm an atheist before my first cup of coffee." - Chuck Colson