How to Apply Gelcoat

how to apply gelcoatBelow you will find a detailed gel coat preparation guide on how to apply gelcoat to your boat RV or other surface. Take steps to cover and protect the rest of the boat before starting. When working on the deck or cabin, tarp off the adjacent areas. 3M and UV tapes , and masking papers are recommended.

Prepping the Boat

    • Remove rails, cleats, louvers, snaps, striping tape, etc.
    • Duct tape off adjacent gunwale molding, and deck fittings you are unable to remove. **NOTE** Duct tape is recommended over masking tape because it provides better protection.
    • Remove seals from the edges of parts or fittings when doing a repair around that part or fitting.

Sanding Gelcoat
The heavier fast-cut grits (40/80/100) are used to feather sand and ground out a routed area prior to filling. Also, they are used for the first sanding of gouges, dock dings, scratches, and blisters. When sanding areas that have been filled with putty we suggest using 40 or 80 grit sandpaper, depending on how large the repair is. Once the fill area is level or contoured to the desired shape, sand with 100 grit paper to remove the course scratches from 40 or 80 grit. You should also feather the surrounding area of the gelcoat repair with 220/330. At this point, the repair is ready to prep and spray/brush with gel coat. When sanding flat areas use a rubber block. Use 3M Abrasives for best results.

Gel Coat Surfacing Agents

Sea Hawk 8140 Wax Additive: Add up to 4oz of 8140 Wax Additive per gallon for tack free surface on the final coat, or if applying one coat a recommended film thickness.

Sea Hawk Patch Aid 8185: Add 25-33% Sea Hawk Patch Aid 8185 when applying gelcoat for small repairs, to improve the working properties of traditional gel coat spray patches. Patch Aid is an additive that significantly improves the application, appearance and longevity of Sea Hawk Gel Coat patches, and also gives a faster cure, harder patch, superior gloss, consistent cold weather performance, superior color match, and less halo. Patch Booster also eliminates the need to thin, or add wax to gel coats prior to patching. Do NOT combine with Sea Hawk 8140. Use either one (8140,8145) but NOT both!

Additives

Tinting: Sea Hawk Gelcoat may be tinted using Sea Hawk colorants designed for use in Sea Hawk resins and gel coats. Do not use more than 1 oz tint per quart.

Thinning Gel Coat

Gel coat may be thinned up to 10% with Sea Hawk 7125 Gel Coat Thinner, or 25-33% Sea Hawk Patch Aid 8185. Use one or the other. Do Not combine. Acetone is NOT recommended except in clean up. Always add thinner, tint, or other approved additives prior to catalyzing with MEKp (also known as the hardener). Over thinning will result in an inadequate thickness for full curing. It is always best to start with 5% and work your way up to 10% if necessary.

Catalyzing Gelcoat with MEKP: The catalyst level (MEKP) should not exceed 3.0% or fall below 1.2 for proper cure. Ideal range is 1.8% @ 77°F. Gel time at 1.8% MEKP is 10-17 minutes. This time element is dependent on material temperature, room temperature, humidity, air movement, and catalyst concentration. Gelcoat should not be used when temperature conditions are below 60°F, as curing may be adversely affected.

MEKP Catalyst Levels

Spraying Gelcoat
Gelcoat may be thinned for use in a gel coat spray gun or preval sprayer with MEK gel coat Thinner. Acetone is NOT recommended except in clean up. Do not use more than 10% by volume of thinner, and always add thinner, tint, or metal flake first then add the Catalyst or Hardener and mix thoroughly. Over thinning will result in an inadequate thickness for full curing.
Please note

  • Gel Coat requires the addition of fiberglass resin hardener or catalyst (MEKP) at 1.8% by volume (77ºF) (approximately 12 drops per ounce)
  • Measure catalyst accurately. Under or over catalyzation retards curing and causes fading and chalking
  • Gelcoat will not fully cure without adding a surfacing agent or wax additive sanding aid. You can over-spraying with PVA before gel coal reaches its gel stage (5-10 min.), or adding 1oz per quart of wax addtiive sanding aid.
  • All Gelcoats from the Bottom Paint Store come with the MEKP catalyst and wax additive sanding aid, but additional amounts can be purchased.
  • Gelcoat should not be applied over paint, wood, metal or concrete. It usually will not adhere to these surfaces.

Initially spray a cover coat as smooth and evenly as you can to cover your repair. This coat should be mostly in the repair area, and may be repeated. Sanding between coats is not necessary unless a surfacing agent has been added to your gel coat. Once your repair is covered, feather a ‘flow’ or sanding coat on the masked-off area making layered passes to avoid a buildup of gel coat in any one area. Each of these coats of gelcoat may involve several passes. Be even and consistent in your gun movement, overlapping each previous pass slightly and not hesitating on the ends. When spraying to a radius, flow the gelcoat to the tape. In open flat areas, layer passes. Final thickness should be 16-20 mils minimum or the gelcoat may not fully cure. As soon as you are done spraying, clean your sprayer fully with acetone. Most re-sprayed gel coat will cure in 2-4 hours, although overnight cures are ideal. Begin wet sanding with the finest grit that will remove orange peel in the re-sprayed area. This will avoid unnecessary sanding scratches. Usually 320 or 400 grit wet paper is sufficient for the initial sanding. Wet sand to a 600 finish and buff with 3M Finesse-It Marine Compound and polish with Scotchgard Marine Liquid Wax.

Brushing Gelcoat

The major disadvantage of polyester coatings is that they cannot be applied in a perfect “self leveling” coat. It must be mechanically finished by wet/dry sanding and polishing if a factory mold finish is to be expected. Sand the entire surface to be coated with 150 grit sandpaper. If cracks or gouges are present, they should be repaired first. When sanding gelcoat is completed, the surface should be free of flaws and perfectly smooth. Choose a good pure (natural) bristle brush with tapered ends. Avoid brushes that are either too stiff or too soft. For most work, a 3 or 4 wide brush will suffice. If there is a trim color, you should have a narrow trim brush on hand.

DO NOT WORK IN DIRECT SUNLIGHT
Choose a shady location or an overcast day. You should catalyze your material so that it cures as quickly as possible within your working time. Generally mix one-pint batches. A good practice is to pour the mixed gel coat from the mixing container into another container used for application. This further assures that no uncatalyzed material is clinging to the sides of the pot. “Lay on” the gel coat in a heavy thickness (about 10 mils) using horizontal strokes, and working from top to bottom. Avoid re-brushing as this could remove the waxy surface additive. Remember to always lap wet. Generally, one heavy coat is sufficient. However, if you have opacity problems two coats may be necessary with a light sanding between coats. Once the coating (gelcoat) has cured, it should be block sanded using a 250 wet grit paper to remove all brush marks and high spots. Following this, it should be wet sanded with 320,400 and 600 wet grit paper, buffed, polished and waxed.

Important Tips

1. Clean repair area and all tools with acetone prior to application

2. Pot life (amount of time for application of the product) shortens dramatically as you mix larger batch sizes. Keep in mind that you have a limited time to apply what you have mixed (usually between 5-15 minutes).

3. Measure catalyst accurately. Under- or over-catalyzed gel coat will cure slower and look faded or chalky.

4. Do not add more than 10% by volume of MEK gel coat Thinner.

5. Gel coat will not fully cure without adding a surfacing agent or over spraying with PVA

6. Do not work in direct sunlight

7. Check color thoroughly before applying. Gelcoat will not darken or change colors when it dries. The color wet is the color when dry.

8. Store gel coat in a cool, dry place

9. Gel coat should be applied in temperatures of 60ºF to 80ºF

10. Always use eye and hand protection

11. Read all warnings on product labels

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