I suppose it depends what you mean by paint. If eg I sent my calipers off for professional stripping, powder coating and rebuilding. If they failed I'd expect the warranty to be with whoever did the rebuild. Trub is, if you spray them yourself ... or use a brush and some hammerite. I've seen people get paint between the
guide pins and when it's dried it's hindered the pins moving. On inspection, I can't imagine them paying out on warranty. Having said that, I'd still expect them to honour it on the rest of the car, pads, discs, etc.