Gelcoat Calculator
How much gelcoat a part, mold, refinish or repair needs, the MEKP to add at your shop temperature, and whether the last coat needs wax. The answer updates as you type.
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- Spray or roll
- MEKP for gelcoat
- Waxed or unwaxed
Gelcoat coverage chart
Square feet one gallon of gelcoat covers at each total wet film thickness. Spraying loses about 30 percent to overspray; brushing and rolling lose about 15 percent.
| Total wet film | Typical use | Sq ft per gal, no waste | Sprayed | Brushed or rolled |
|---|---|---|---|---|
| 10 mil | Thin touch-up coat | 160 | 123 | 139 |
| 15 mil | Minimum for a finished part | 107 | 82 | 93 |
| 20 mil | Standard part in a mold | 80 | 62 | 70 |
| 25 mil | Heavy part coat | 64 | 49 | 56 |
| 30 mil | Rolled refinish, before sanding | 53 | 41 | 46 |
| 40 mil | Mold surface (two 20 mil coats) | 40 | 31 | 35 |
1 mil = 0.001 in (0.0254 mm). Coverage = 231 cubic inches per gallon / (144 x film in inches).
MEKP for gelcoat by temperature
Gelcoat runs a higher catalyst dose than laminating resin. Measure in cc with a graduated dispenser.
| Shop temperature | Dose | Per pint | Per quart | Per gallon |
|---|---|---|---|---|
| 60°F and below | 2.5% | 12 cc | 24 cc | 95 cc |
| 65°F | 2.35% | 11 cc | 22 cc | 89 cc |
| 70°F | 2.2% | 10 cc | 21 cc | 83 cc |
| 77°F (standard) | 2% | 9.5 cc | 19 cc | 76 cc |
| 80°F | 1.9% | 9.0 cc | 18 cc | 72 cc |
| 85°F | 1.75% | 8.3 cc | 17 cc | 66 cc |
| 90°F | 1.6% | 7.6 cc | 15 cc | 61 cc |
| 95°F and above | 1.5% | 7.1 cc | 14 cc | 57 cc |
Percent by volume, held between 1.5 and 2.5 percent. Laminating resin uses a lower dose: see the MEKP catalyst calculator.
How to apply gelcoat
Gelcoat is a pigmented polyester surface coat. It is thicker and harder than paint, and how much you need depends on the film you build and how you put it on.
- In a mold. Wax and release the mold, then spray unwaxed gelcoat to about 18 to 20 mil in two or three passes. Check the film with a mil gauge. Laminate the first skin coat once the gelcoat is tacky but no longer transfers to a glove.
- For a new mold. Spray two coats of tooling gelcoat at about 20 mil each over a waxed plug, then back it with tooling resin and mat. Orange and black tooling gelcoats show thin spots while you spray.
- Refinishing a boat. Sand the old gelcoat to 80 to 120 grit and wipe with acetone. Roll and tip three coats of waxed gelcoat, or add surfacing wax to the last coat. Wet sand from 320 up and polish.
- Chips and gouges. Grind the damage to clean, sound laminate, fill with waxed gelcoat or gelcoat paste a little proud of the surface, cover with film or let the wax cure it, then sand flush.
- Thinning for a spray cup. Thin about 10 percent with styrene for a small sprayer, never with acetone.
Worked examples
| Job | Film | Gelcoat, with waste | MEKP at 77°F |
|---|---|---|---|
| A 25 sq ft part, sprayed | 20 mil | 1.6 qt | 31 cc |
| A 40 sq ft mold face, sprayed | 40 mil | 1.3 gal | 98 cc |
| A 60 sq ft deck, rolled | 24 mil | 1.03 gal | 78 cc |
| Six 2 sq in chips, 1/16 in deep | fill | 0.7 fl oz | 0.4 cc |
Gelcoat calculator FAQ
How much gelcoat do I need?
Multiply the area in square feet by 144 and by the total wet film in inches, divide by 231 for gallons, then add waste: about 30 percent when spraying and 15 percent when brushing or rolling. At a standard 20 mil film, one gallon covers about 80 square feet before waste, or about 62 square feet sprayed.
How thick should gelcoat be?
A finished part wants about 18 to 20 mil of wet gelcoat, built in two or three passes. A mold surface wants about 40 mil, usually two coats of 20 mil. Under 15 mil the gelcoat can stay soft and show the fabric pattern, and more than about 30 mil in one coat can sag and crack.
What is the MEKP ratio for gelcoat?
About 2 percent by volume at 77°F, which is roughly 19 cc per quart or 76 cc per gallon. Use a little more in a cold shop and a little less in heat, and stay between 1.5 and 2.5 percent. Measure with a graduated dispenser, not drops.
Do I need waxed or unwaxed gelcoat?
Use unwaxed gelcoat in a mold, because the laminate behind it needs to bond to it. Use waxed gelcoat, or add surfacing wax at 1/2 oz per quart, for any coat that cures in open air, such as a refinish or a repair. Without wax the surface stays tacky.
Can gelcoat go over epoxy?
No. Gelcoat is polyester and does not cure or bond reliably on epoxy. Put it over sanded polyester or vinyl ester laminate or old gelcoat. For an epoxy repair, finish with a paint system made for epoxy instead.
How many coats of gelcoat should I roll on a boat?
Three coats of about 8 mil each is a good target. That builds roughly 24 mil, which leaves enough film to wet sand the roller texture flat and polish without cutting through.
How long do I have to work with catalyzed gelcoat?
About 15 to 20 minutes at 80°F at the standard dose. Catalyze no more than a quart at a time, or half a quart when the shop is above about 85°F, and keep the can out of direct sun.