What it all comes down to is this: waxes and sealants are designed to bond to paint, not the dirt on the paint. Further, gloss comes from reflectivity, and you can't get good reflectivity off of dirty paint or paint that isn't really flat. So you basically have two types of major defects in paint; above surface contaminants and below surface defects.

Above surface bonded contaminants are things like paint overspray, industrial fallout, tree sap, etc. Basically anything that does not come off when you wash the car. This is what a clay bar is designed to remove. Used with a lubricant like a quick detail spray or soapy water (the quick detail spray is the best though) a clay bar will gently remove the bonded contaminants that make your paint feel rough to the touch. It does nothing for swirl marks, scratches, etc and will not damage the paint or remove any clear coat. While there are various grades or abrasiveness levels of clay, all of the popular over the counter clays (Meguiar's, Mothers, etc) are very mild. The heavy duty stuff is mostly for body shop or pro detailing use where serious issues exist and they will micro mar the paint, but a full polish is always expected to be done after using these clays so it's a non issue. The process of claying is quite simple in itself though, and takes very little effort. You don't need a lot of pressure so it doesn't wear you out. Just remember that if you ever drop the clay on the ground to throw it away immediately. NEVER allow that dropped piece of clay to touch your car ever again.

Below surface defects are things like swirl marks, scratches, etchings from bird droppings, eggs, acid rain spotting, etc. The only way to remove these is to polish the paint using some sort of abrasive product that does, in fact, remove a very small amount of clear coat. I've seen numerous tests using paint thickness gauges showing fine swirl marks being removed by high speed rotary that indicate the amount of clear removed is so negligable that you shouldn't be concerned at all - at least when the rotary is used by a skilled detailer. Nonetheless, this is the ONLY way to actually remove these defects. There is also the issue of what is called "dirt in paint" and this is exactly what it says. Over time a vehicle left outside, even if regularly waxed, will actually get dirt embedded in the paint. A paint cleaner, which is really just a very mild abrasive polish, will remove this. It can be quite noticeable on lighter colored cars but since it accumulates slowly over time you might not even notice it. Until you start working on the paint, then it sort of slaps you in the face.

So what you ultimately want to do is wash and dry the car then critically evaluate the paint under very harsh light. Direct sunlight is good for this, as are halogen lights. You're looking for swirl marks and other defects. Also, slip your hand into a plastic sandwich bag and lightly drag it over the paint. It should feel as smooth as a new bathroom mirror. If not, it's time to clay. You'll want to clay before you polish so as to remove all those above surface contaminants first, then you can attack the below surface defects. If you clay and polish properly you'll end up with a finish that looks absolutely stunning, and you haven't even waxed it yet!! Now apply your favorite wax or sealant, or combination of the two, and you're set.