Gelcoat Application

There are a lot of differences between Gelcoat and a good paint finish.



 Things to consider in choosing a Polyurethane paint or gelcoat'

I have discussed the pros and cons of paint and/or gelcoat in another page.

What really needs to be explained is whether or not you can even use gelcoat to begin with. If the boat has a paint finish already on it, you cannot put gelcoat over it. It all needs to come off right back down to the original gelcoat and maybe fiberglass.

As far as preparation goes, you will be facing pretty much the same amount of work ether way. Only difference with gelcoat is that much of that labor comes after the application of the gelcoat.

If there are repairs to be made you must do them in polyester resins. Gelcoat will not stick to epoxy.

Now we need to determine if the boat needs to be faired. Fairing is a long and laborious job that is meant to fill-in larger areas so that when you view the finish at an angle, you don't see wavy spots or "oil can" dents in the hull. Fairing is done exclusively on vintage cars. Look at a door on a nicely restored car and you won't see any waves or dimples anywhere. If you are needing to fair the surface, you need to decide to what extent you want the surface to look like. Certainly on a boat we don't need to reach the standards of Concourse Cars. But in my case with Flying Cloud I knew that I would never have the opportunity to fair her in the future. She was in bad shape and of course I went much farther than was really needed on a cruising boat.
But I think the photos show the proof of my labor. UNTIL I hit the first pilings anyway!


if you want to more about fairing, click on this link  FAIRING

The main difference between gelcoat and paint is that for paint, the primer is sanded and double checked for mars, sags or other problems and then the topcoat is applied by spray or roll/tip. Once cured, you are finished.
So its about the same amount of work either way. Its just the sequence of events that are reversed.
gelcoat will need to be heavier in thickness for a couple reasons. Gelcoat does not level out like paint. Even with special additives to thin the viscosity, a certain degree of texture or "orange peel", will be present after cure and will need to be sanded, sealed and polished.
Gelcoat is normally sprayed on in multiple coats to achieve enough thickness to allow for the initial sanding and polishing.

The cured thickness you want prior to initial sanding is a minimum of 30 mils and max 35 mils. This will allow for a final thickness around 20 mils. Any heavier than 25 mils and you run the risk of spider cracks in a few years.

Normal maintenance can reduce the thickness by 2 to 3 mils a year. If you compound and polish it annually.

You should be able to spray 10 mils per application. Allowing for shrinkage in the initial cure, this would be about 4 coats, or;

   1. 10 mils gelcoat with little or no additives and no surfacing wax
   2. 10 mils gelcoat, same as above.
   3. same as above but with a small amount of surfacing wax.
   4. 10 mils with additives and surfacing wax.

Allow the coats to harden for several hours between coats. Coats 1 - 3 will feel tacky after hardening to a point that you can't leave an impression with your finger nail.

Coat 4 should have no tacky feeling at all because the surfacing wax will migrate to the surface as the cure cycle takes place. This is important because you can't sand it until it cures completely.

Gelcoat is very thick and  in order to spray it must be thinned. The less you thin it the better. But I use a 2.2 millimeter tip on my HVLP gravity feed gun.

NEVER thin gelcoat with acetone, MEK or lacquer thinner. The best product seems to be a product called "patch booster" and is available from a number of companies that manufacture resins. 10% to 15% is all you need.

On the third coat add about 10% surfacing wax and let it cure for about 24 hours. Now it can be sanded with 180 grit to remove all orange peel. This is also the point at which you would start fairing and making the surface smooth and straight.

The final coat will be done with a 1mm tip adding between 30% to 40% patch booster and 10% surfacing wax.
This application will be more like spraying paint. because this is thinner, watch out for sags and runs. Allow a couple minutes between coats to get about 10mils. Spray the about 5mils with the first coat and wait and recoat in 2 to 3 minutes.

Let this cure about 24 to 48 hours.  Wash the surface with naptha and a concentrated liquid laundry soap.

Now start sanding with 320 grit paper.  I prefer wet or dray paper because the water keeps the gelcoat from clogging the sandpaper. If you don't feel you are making headway, switch to a slightly heavier paper like 220 or so.

Once the entire surface is sanded to the point that you can't see any shiny spots (when looking at it dry) you are ready to start working down to the final finish. Go from 320 to 400, on down to 800 grit and finally finish up with 1000 grit wet.

This is about the same amount of labor as sanding all that primer when preparing for paint.

The main point here is to look at the dried surface and be sure that all the previous sanding marks from the heavier sand paper, are gone. If everything has the same look that the 1000 grit does, you are now ready to polish.
there are a number of polishing/buffing compound available. But we prefer a product called Aqua Buff. It comes in two different grits and is available online.
Start polishing with the coarse grit and once the entire boat is polished follow up with the fine grit.

Buffing and polishing is generally interrupted as polishing on an d wiping it off. NO! Wax on, wax off was only in a movie.

Polis is small areas and keep the pad moving around slowly so as not to burn the surface. No need to keep adding compound when you see it starting to shine. Basically buff until you see the gelcoat starting to shine. Keep buffing more without adding more compound.
Once you see that your efforts are not making in difference, move to another area and repeat until the entire boat is free of the sandpaper marks.

On gelcoat we like to use Mequires products. Keeping the gelcoat waxed is very important.


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