Appropriate dress attire

The Barcelona cathedral has a knees and shoulders covered policy, and I have seen it enforced. I've heard that other churches do as well, but have not seen it enforced in, say, Seville or Granada. I've seen this much more in Italy. Regardless, I'd make sure to pack pants/long skirt and t-shirts or something other than tank tops just in case.

This would only apply in churches. To the best of my knowledge, other sites like museums, palaces etc do not have a dress code

Just wondering if you have a sleeveless blouse on would putting a scarf over your shoulders before entering church be consider proper attire?

Yes Sue... know though that despite some guards at the Cathedral in Barcelona can be overzealous, unless you're dressing up BritneySpears-style you shouldn't have much problem visiting it, or any other religious building here in Catalonia for that matter. Yet carrying a scarf or something can be a good idea just in case. I can't vouch for other places though, especially in the South of Spain, as they're notoriously more "religious" than us here.

Tip: should you have any problem entering the Cathedral you can do two things: either try thru the side entrance (another guard, another interpretation of the rules) or else come back later (another shift, another guard). Yet, in general terms, only the blatantly unfit outfits are really banned there.

As per dressing elsewhere, most of the time there is no established dress code albeit for certain places (ie. opera, certain upscale restaurants, special events...) a more formal dressing might be required. Yet in general terms the Barcelona-style is mostly smart casual. Note that on any given summer day you'll find a mix of styles because some "visitors" mistake the city for a summer resort and insist on the shorts and flip-flops, LOL! :))

Yes, a scarf over your shoulders would work. I suppose some overzealous guard could have issue with a sheer scarf, but a normal opaque one would be fine.

Barcelona cathedral is the only place I have seen this enforced in Spain. I'm sure if you were in some sort of really revealing outfit you'd have trouble in more places. But the usual shorts/skirt a bit above the knee and sleeveless blouse are likely to be fine. Again, bring something more modest just in case.

Thanks everyone. I should of mentioned I will be on the Village Italy tour first two weeks of September . I am concerned about the heat..
I packed one scarf to always be in my backpack. A few of my blouses are sleeves but not skinny tanks..I posted here because I saw the question here.
Thanks,
Sue