3 coats of epoxy after fairing and sanding - is it a must?

I am a first time boat (skiff) builder. I have just built an 11 ft. skiff and am preping it for painting. After having fiberglassed, epoxied, then epoxied some mure, and sanded the hull, I applied the fairing coat (MAS microballoons) to the exterior of the hull and sanded it down so that the hull is very smooth. The plans indicate to apply 3 coats of epoxy and sand again before painting. Given all the sanding that would need to be done after the 3 coats of epoxy, I am curious as to the benefit of these 3 coats of epoxy? I have read about barrier coats of epoxy for moisture proofing, etc., but not sure how much of that benefit remains after the sanding. If the epoxy is a recommended must, then what grit of sandpaper is recommended (I am using an orbital sander).

thanks,
Ctrek

 

What skiff. What plans? I'm guessing stitch and glue plywood. If it were some levels up it would be glued lapstrake and you wouldn't be using an orbital. Did you use Shelmarine plywood for the "planking"? This designer is telling you to really protect the plywood in there because if water does find it's way, you've got a plastic bag with wood and water in it...in Florida! The first step IMO, is to use the best plywood available so that if it gets wet it's no big deal. If you are finishing bright and like swirls, BS1088 Meranti, If covering with goo anyway, Shelmarine, as it has a fungicide and will last decades, or more, no matter what you do to it. Since you are already past the point of "which wood", I recommend following the designer's instructions unless you really baby this boat or don't mind a ten year lifespan. Consider sacrificial strakes on the bottom, maybe with half-round brass.

 

The three coatings are a must to remain waterproof in average use over a long period. If you apply a filler after sanding 120 grit (dry) will do. If you sand wet use 160 grit, same if you do´nt use a filler, but you have to sand the first coat of paint then with 200.

Regards
Richard

 

The method of building is the basic stich and glue. Given the recommendation to apply 3 coats of epoxy and then sand it down as preparation for sanding, what remains of this barrier coat if I have to sand it down prior to painting?

 

Depending how fair you applied the glass, but not much.

 

If the sealer coats are applied in succession, no sanding is necessary. A chemical bond will be achieved when recoated before full cure.
Three coats is just an average based on the kind of thickness you'd apply on a suface that's vertical and might run. One coat is all that's needed if the surface is dead level, but it's a thick one.
If you work clean, little final sanding is needed. Often, like amateur painters whoi are also perfectionists, people who aare unfamiliar with epoxy use get a lot of dust and crap into the mix. You should treat it like a paint job and remove all dust. The result will be that epoxy is like paint---- it makes a perfect finish if the one applying it knows what to do.
The sanding you do after "three coats" will depend on the paint's requirements. A heavy-bodied primer allows you to sand with 150 and then you can wet sand the primer with, say, 320, and apply two finish coats.

 

Use cure time properly

Hell Ctrek

Alan is right in using the cure time of epoxy to avoid sanding. With the place very clean you put your first coat on and then when it is tacky (leaves a thumb print) put your next coat on. If you roll it on with a foam roller two is enough. If you use a squeegee you will need more.

It is truly impossible to say how many coats you need. If you have some wavy grain then the epoxy gets soaked up so you need more. As you have fairing compund over yours it should be pretty homogenous.

BUT

Do not think you have finsihed fairing - you haven't. You have macro faired the project. If you want a really good finish you must start thinking differently now the epoxy is on. You need a couple of coats of epoxy high build primer over the lightly sanded barrier coat. Then you smooth this down with 180 then 240, 320 or whatever your painter says. Fairing compound takes out the wobbles in the boat you can feel. High build takes out the very small imperfections you can only see when you spray.

You know you are getting close when the hull looks really nice (but a little dull) when wetted. Don't rush the last part or you will end up with builders lament. This is where they get very disappointed with their boat as soon as the paint is on. It takes days to justify to yourself why it doesn't matter.

cheers

Phil

 

The three coats over the fairing compound seals the compound, which will absorb moisture and lift from the surface. Apply each coat just after the previous coat has lost it's tackiness and you don't need to sand between coats. Of course the last coat should be left to fully cure then you begin the prep for paint process.

 

That include west 410 Par?
Also, I flow coated my sealer (hull upside down) at 0.75l/m^2 with fast hardener & shed heated to 22deg C.
I can roll more on upside down prior to priming.

 

West System Microlight (410) is a micro balloon mixture and really needs to be sealed, especially under the LWL. Depending on the paint system you'll use, you can often skip the primer, if you have the surface smooth enough. This said, most can't get a smooth surface without "blocking" down a bunch of primer. Primer also provides an even color base for the topcoats so you don't end up with blotchy areas. If your boat needs a good bit of smoothing, use a "high build" primer. If the primer is epoxy based, then get all your sanding done within 72 hours or less, otherwise it'll be a lot harder.

 

Remember that microballoons are tiny hollow glass spheres. When you sand them the spheres are broken leaving little pockets for water ingression. If you wet sand, be sure to allow enough time between sanding and re-coating to be sure that all the water is gone from the microballoons. If you do not, nothing you apply subsequently will have really good adhesion. You can make totally waterproof filler by using 'microspheres' which are solid rather than hollow, but they are substantially heavier. A gallon of microballoon filler weighs about 8 lbs and a gallon made with microspheres weighs about 12 lbs. This adds up when you do a lot of fairing on a boat.

Jimbo