A hollow is not a box
The instinct is to multiply length by width by depth. For the hollow in the form — 30 in by 14 in with a 0.18 in gap under the straightedge — that box holds 75.6 cubic inches. The hollow does not, because it tapers to nothing at every edge. A rounded dish holds about two thirds of its box, so 50.4 cubic inches of compound ends up staying on the hull.
The page prints all three shape factors side by side so the size of that decision is visible: 0.592 of a gallon to mix if it is a flat-bottomed sink, 0.395 if it is a rounded dish, 0.197 if it is a sharp dent. Same three measurements, three times the material between the extremes.
Most of what you mix comes off again
This is the part that separates fairing from filling. You do not trowel compound up to the fair line; you trowel it past the line and then find the line with a long board. At 35 percent sanded back, the 50.4 cubic inches that stay require 77.5 cubic inches applied, and the 15 percent that never leaves the board or the pot takes the mixed volume to 91.2 — 0.395 of a gallon, three and a half pounds on the scale.
That also drives the layer count, which is why the page works it from the applied depth rather than the measured gap. The deepest point has to be built to 0.277 in before sanding, not 0.18, and at a 0.09 in maximum per pass that is four layers rather than two. Anyone who has planned a fairing job as an afternoon and lost a week to it has met exactly this arithmetic.
Elapsed time is not working time
Four layers at 25 minutes each is one hour and forty minutes of actual trowelling. Three cure gaps at six hours each is eighteen hours of waiting. The job is a bit under twenty hours elapsed and under two hours of work, and the whole difference is standing about.
In a heated shop those cure figures hold. In an unheated shed in March they do not, and this is the single number most likely to be wrong on the page — the datasheet quotes cure at a temperature, and the temperature in the field is whatever it is. Everything downstream of a stretched cure stretches with it.
Gelcoat is a different animal
The gelcoat section takes a coverage per quart rather than deriving one, because gelcoat is applied far thicker than paint and the useful number is on the label. Six square feet at 20 sq ft per quart over three passes is 0.9 of a quart, so one quart bought.
What no calculation touches is the colour. A hull that has been in the sun for fifteen years is not the colour it left the mould, and a perfectly mixed batch of the original gelcoat colour will show as a patch. Matching is done by spraying out test cards and living with the result, and it is the part of a gelcoat repair that people underestimate by the widest margin.
Questions people ask
How much fairing compound do I need for a hull hollow?
Measure the length, the width and the biggest gap under a straightedge, then multiply and apply a shape factor — about two thirds for a rounded dish. For a 30 by 14 in hollow 0.18 in deep that is 50.4 cubic inches kept on the hull, which becomes 0.395 of a gallon mixed once you allow for what the long board takes off and what stays in the pot.
Why does a fairing job use so much more compound than the hollow holds?
Because fairing is applied proud and cut back to a line found with a long board, so a large share of what goes on never stays. At 35 percent sanded back plus 15 percent left on the board and in the pot, the mixed volume is about 1.8 times the volume of the hollow itself. Filling a hole and fairing a surface are different jobs with different quantities.
How many layers of fairing compound can I put on at once?
That is set by the compound datasheet and is a field on the form rather than a figure this page supplies. What matters is that the layer count comes from the applied depth, not the measured gap: at 0.18 in of hollow and 35 percent sanded back you are building 0.277 in at the deepest point, which at 0.09 in a pass is four layers rather than two.
What shape factor should I use for a dent?
A rounded dish where the straightedge touches at both ends and the gap grows smoothly to the middle takes about two thirds of the containing box. A sharp impact dent takes about a third. A panel that has sunk evenly between stiffeners is closer to flat-bottomed and takes the whole box. The page prints all three so you can see how much the choice is worth.
How much gelcoat does a repair take?
Area times passes divided by the coverage on the label, which for six square feet over three passes at 20 sq ft per quart is 0.9 of a quart. Gelcoat goes on much thicker than paint, so the coverage figure is far lower than any paint figure. The volume is the easy part — matching the colour of a hull that has been in the sun for years is not.