Rubbing out spends film
A rubbed finish is flat because the abrasive has taken the high spots down to meet the low ones, and the low ones are the film you already had. That is the whole mechanism. Levelling a brushed film with visible texture costs more than levelling a sprayed one that went on nearly flat, and every polishing step afterwards costs a little more on top.
Work a case through. Four coats at 1.2 mils gives 4.8 mils of film. Levelling takes 0.8 and three polishing steps take 0.15 each, so 1.25 mils comes off in total: about a coat, and roughly a quarter of everything you built. The field is left with 3.55 mils and that is comfortable. The edges are where it stops being comfortable.
Edges hold less film than the field
Finish is a liquid with surface tension, and on a sharp arris it flows away from the corner while it levels. Nobody can tell you what fraction remains without measuring, but it is always less than the field, and the sharper the edge the worse it is. Enter 60 percent and the same four coats leave 2.88 mils on the edge before rubbing out, and 1.63 mils after, which is under a 2 mil working minimum by a third of a mil. One more coat at 0.72 mils of edge build closes it.
This is why breaking edges before finishing is not just about feel. A radius holds film where an arris sheds it, and it is also far more forgiving when the abrasive goes over it, which happens whether you intended it or not.
The minimum is yours
The page asks what film you want left and then reports against that number, because there is no general answer. What film a product needs to do what it was designed to do is a manufacturer question, and the honest way to use this page is to decide your own working minimum from your own experience and see whether the schedule clears it.
Where this sits
The build side of the equation comes from the film build calculator, which turns volume solids and wet film into film per coat. The elapsed time the coats need before you can start levelling is in the finishing schedule calculator, and a film sanded before it is hard enough loads the paper instead of cutting. For the abrasive sequence itself, the grit progression calculator covers what each step has to remove and how long it takes.
Questions people ask
How do I find out how much film my levelling pass removes?
Measure it on a test panel, because it depends on your paper, your block, your pressure and how flat the film was to begin with. The straightforward method is to finish a panel the same way you will finish the work, level part of it and leave part of it alone, then measure both. Without a film gauge, weight works as a proxy: weigh the panel before and after and convert, since you know the area and roughly what the cured film weighs per unit volume. Either way the figure is yours and it is stable once you have it.
Why does the calculator care about edges separately?
Because that is where cut-through happens, and the field figure hides it completely. A panel with 3.5 mils left in the middle can be through to bare wood a quarter inch from the corner, and the flat number tells you nothing about that. Modelling the edge as a share of the field build is crude, but it makes the risk visible before the abrasive does, and adjusting one number lets you see how much a broken edge or an extra coat buys you.
What happens if I do cut through?
You get a pale witness line where the abrasive reached the wood or the stain under the film, and it does not polish out because there is nothing left to polish. On a stained piece it is worse, since the colour goes with the film. The repair is usually to stop, let everything harden, and recoat the whole surface rather than the spot, because a patch of new film over an old one shows as a difference in sheen even when the colour matches perfectly.
Does rubbing out need more coats than a finish left as sprayed?
Yes, and that is the trade rubbing out asks for. A finish left as it came off the gun keeps every mil you put on. A rubbed finish gives back whatever the levelling and polishing take, which this page adds up, so the schedule has to build that much extra before it starts. That is the argument for spraying if you intend to rub out: a flatter starting surface means a smaller levelling allowance, and the coats you save are real ones.
Can I rub out any finish?
Whether a given product can be levelled and polished at all, and how long it has to harden first, is a product question that belongs on the technical data sheet. What is general is the arithmetic: whatever the product, the abrasive removes film, the edges start with less of it than the field, and a schedule that does not account for the difference finds out on the last step of the job rather than the first. Run the numbers before the last coat rather than after.