Well, the whole lead-blocks-X-ray-vision idea is based on the fact that lead blocks X-rays. Since lead alloys should block X-rays to some extent depending on their lead content, it's safe to assume Clark couldn't see through lead alloys, either. This plot bunny has been used occasionally, where he cannot see through things painted with lead paint.
To be accurate, though, there are two facts to mention:
1) Lead is not the only substance to block X-rays and/or radiation, although in the Superman-universe, it's the only stuff he cannot see through and which can block kryptonite radiation.
2) Clark's X-ray vision doesn't have anything to do with X-rays, or he wouldn't be able to literally see colored images through walls.
