Everyone has it. The most common term is rail dust but what it is is embedded iron particles. A clay bar will not remove them...it just cuts it off at the surface and even if you coat it the spots will return. The only thing that gets rid of it is specialized products like Iron X which you can get off eBay. Spray that on, you’ll see all the spots turn purple then rinse it off then wash like normal. These are normal contaminates in paint and you’ll see them again in the future...white is hands down the worst color for them.
My last 10 vehicles have been white. I’ve had my share of experience with this very problem.
I use IronX and and a variety of other products to decontaminate paint. I typically use both a decontaminate and a clay when doing a complete detail especially on a brand new or new to me vehicle.
I personally prefer clays to the clay mitts but to each their own. Valuegard ABC 3 step is hands down the most effective product I’ve used to decontaminate a finish. It’s not really expensive but the initial expense is a bit pricey for the average hobbyist (about $50 for perhaps a dozen uses). Finish Kare also makes a 3 step that works though I was not as impressed with it. Then their are the one steps like IronX and a bunch of others that fall into this category, they also do what they claim though aren’t an end all miracle product.
While the liquids are of great worth in getting in tight places (wheels spokes and such), whole fenders, panels or places that do not require a lot of ”scrubbing” I find that clay is more effective for most spot blooms.
The suggestion/myth that clays or like products simply sheer the particle off at paint level has been around for years. While there likely is some truth to this clays will also drag/pull particles out completely. Make sure you knead your clay often.
There is also the suggestion or myth that decontamination liquids will often not completely dissolve the larger particulates in the recommended dwell time. They sit on top of the paint and dissolve rust. If you’re leaving this stuff on longer than recommended time frames be careful, you may be inviting problems. At the very least the old adage of lather, rinse, respect (for lack of a better term) is wise.
Discounting the clay products categorically is a lot like throwing the baby out with the bath water. I certainly believe there is a time and a place for both a decontaminate and clay. I’d suggest the OP give both a shot and see which works best for him.
YMMV