The same green, two films: 7 to 12 mil cured at the plant, 4 to 5 mil out of a can. Both in micrometres, with the square feet per can and the 2% / 5% repair limits worked out per foot.
← Back to the calculatorIn A775's own wording the plant figure is the band at least 90% of recorded readings have to fall inside, not every point on every bar. The aerosol film is what a cut end or a dragged bar gets after the fact, and one can covers 8 to 9 ft² at that film.
Everything below is unit work: mil to micrometre, square feet of can to feet of bar, percent of surface to square inches per foot. The two film numbers come from the standard and from the coating maker's own data sheet, and the arithmetic is shown so you can redo it for a bar size that is not in the table.
What the page does not do is treat the can as a substitute for the plant. A 4 to 5 mil patch over a damaged spot is what D3963 asks for; a bar that was never coated at the plant is not repaired by spraying it.
The colour is identical, which is why the two get confused. The film is not.
| Film | Dry film thickness | In µm | Applied | Governed by |
|---|---|---|---|---|
| Plant fusion-bonded epoxy | 7–12 mil | 178–305 | Powder, cured in an oven | ASTM A775 |
| Aerosol touch-up | 4–5 mil | 102–127 | Spray can, air dry | ASTM D3963 |
| Aerosol, one coat | 1–2 mil | 25–51 | Per pass, before build | Maker's data sheet |
Swipe the table sideways on a phone.
The conversion is one line, and it is the line nobody prints: 1 mil = 25.4 µm, so 7 mil is 177.8 µm and 12 mil is 304.8 µm. The can's own data sheet rounds 4.0 to 5.0 mil to “100 to 125 µ”; the exact conversion is 101.6 to 127 µm. Either way, a ratio between the can and the plant film means nothing until the divisor is named. Against the 12 mil upper limit of the plant film the can lays down 33% to 42% of it (4/12 = 0.33, 5/12 = 0.42). Against the 7 mil lower limit the same film is 57% to 71% (4/7 = 0.57, 5/7 = 0.71). Across the whole band the honest figure is 33% to 71%; there is no single percentage that is true at both ends.
The plant number has an acceptance rule attached, and it is the part most often misquoted. ASTM A775 does not say the film must be 7 to 12 mil at every point. It says that for acceptance, at least 90% of all recorded thickness measurements after curing shall be 7 to 12 mil. Ten readings with one at 5 mil can still pass; ten readings all at 13 mil fails on the upper limit.
The aerosol is a repair material, and the specification it is written against is the field-handling one: cut ends, nicks, abrasion from handling, and the holidays that show up after a bar is bent. The maker's data sheet puts the practical coverage at 8 to 9 ft² per can at the recommended film, assuming 15% material loss, and it sets the working window at a surface temperature of 50 to 100 °F with humidity below 85%.
Two numbers on that sheet are worth keeping because they bound where the can can be used at all: dry heat resistance 200 °F, and a hard cure before the bar goes into concrete. A patch that is still soft when the bar is placed is not a patch.
The can is sold in square feet and the bar is bought in feet, so the two have to be joined by the bar's surface area per foot. For a round bar:
With 8 to 9 ft² per can, that is:
| Bar | d (in) | A (in²/ft) | 2% of A | 5% of A | One can covers |
|---|---|---|---|---|---|
| #4 | 0.500 | 18.85 | 0.377 | 0.942 | 61–69 ft |
| #5 | 0.625 | 23.56 | 0.471 | 1.178 | 49–55 ft |
| #6 | 0.750 | 28.27 | 0.565 | 1.414 | 41–46 ft |
| #7 | 0.875 | 32.99 | 0.660 | 1.649 | 35–39 ft |
| #8 | 1.000 | 37.70 | 0.754 | 1.885 | 31–34 ft |
Swipe the table sideways on a phone.
Read the last column as an upper bound, not a plan. It assumes every square foot of the can lands on bar at the full film, with the 15% loss already taken out, and it ignores overspray between bars. On a #4 that is roughly 61 to 69 ft per can; on a #8 it is 31 to 34 ft, because the bigger bar has twice the surface per foot.
The repair limits are written as percentages of surface in a one-foot length, which is not a number anyone can measure on a bar. Converted with the same area formula:
On a #4 that is 0.377 in² of damage and 0.942 in² of patch per foot; on a #8, 0.754 and 1.885. The columns in the table above are those two numbers for #4 through #8, so the arithmetic does not have to be repeated for the sizes in the table.
Two things the percentages do not say. 2% is not an allowance to leave damage in the bar — all visible damage gets repaired, and 2% is the point at which the bar may be rejected. And cut ends are outside the count: a cut end still has to be coated, but the area of that patch never appears in the 2% or the 5%.
Two numbers, depending on where the film was applied. Plant fusion-bonded epoxy is 7 to 12 mil after curing, which is 178 to 305 µm, and ASTM A775 accepts the bar when at least 90% of recorded readings fall inside that band. An aerosol touch-up puts down 4 to 5 mil, 102 to 127 µm, which is 33% to 71% of the plant film depending on which end of that band you divide against.
8 to 9 ft² at the recommended film, with 15% material loss already assumed in the maker's figure. In feet of bar that is about 61 to 69 ft of #4, 49 to 55 ft of #5, 41 to 46 ft of #6, and 31 to 34 ft of #8, because surface per foot rises with diameter.
For repair, yes, and only against the repair specification. The aerosol exists to cover cut ends, nicks and handling damage on bar that was already coated at the plant. It cannot turn uncoated bar into coated bar, and a 4 to 5 mil air-dried film is not the 7 to 12 mil oven-cured film the bar was accepted on.
2% of the surface area in a one-foot length before patching, and 5% of that area covered by patching material, with cut ends excluded from both counts. Those convert to 0.377 and 0.942 in² per foot on a #4, and 0.754 and 1.885 in² on a #8. The 2% is a rejection threshold, not a licence to leave damage in the bar.