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I need to add a few layers of glass on some spots on the outer side of the hull, as, out of so many years of grinding away the antifouling (we put antifouling every year and we even it out after each season) the corners around the have become almost transparent.

I fail to pinpoint the discussion about the material to use; I remember Mike saying that polyester does not stick on polyester and recommended epoxy instead.
Could someone shed some light, what type epoxy is needed and what prep on the hull?
Thanks in advance.

Two types of polyester resin, laminating and finishing. Both do what they say, using MEK as the hardener. Polyester will go over polyester, naturally prep work before, grind, sand, remove dust, wipe with Acetone before fiberglass work. West System Epoxy will go over polyester to, and adhere and work very well, with filler or mat, ( biaxial or heavier mat, woven roven). Going the other way...Polyester will not reliably go over or adhere to West Epoxy very well. EarleyBird

Basic rule of thumb.

Epoxy is an adhesive. Polyester is not an adhesive.

Poured monolithically, they are roughly equal in strength. Polyester resin is much less expensive than epoxy. That's why boat builders use it.

I'm not suggesting that polyester resin isn't sticky. It is. So is chewing gum, but I wouldn't use it as an adhesive. When you want to glue two things together, epoxy will form a much stronger bond.

The only exception will be brand new polyester. So if you make something out of poly and 12 hours later you laminate a new piece onto it, you can use poly for the new piece because you will still get a chemical bond between the two pieces. The two pieces essentially become one. After about 24 hours that becomes impossible, and the only bonding available is mechanical. Rough up the old surface, clean it well with solvent, and use epoxy.

Thank you Earleybird and Mike.

Three things:

Please confirm that the prep work necessary to apply new epoxy over old polyester is IDENTICAL to applying new poly on old poly. It is just the substance to apply that changes, not the prep method.

For those who do not necessarily have west system, could another brand of two ingredient epoxy do the job? There are quite a few epoxy brands, is there anything IN PARTICULAR to look for in purchasing one brand over another?

Do I need to lay gelcoat over the mended surface? The area is under the waterline and will soon be covered with antifouling,
so the patch will not be visible.

Thanks again.

Same prep. Rough up the surface, clean well, go to work.

MAS makes a great epoxy product. I'm guessing that most others are also fine.

You don't need to gelcoat. The inside of your boat is uncoated fiberglass. No harm in it being wet. Paint won't hurt it, antifouling paint included.

In addition if possible, Mike will probably agree, gel coat is not waterproof. If you can follow the instruction on the bottom paint, most require barrier coats before the anti-fouling (bottom paint). Have fun. EarleyBird.

Yannis - Where on the hull? You say "corners around the..." Chines, strakes, transom???

Process is likely the same West Systems has good "how to" articles on line -
- Clean the area
- Grind out to clean glass, feathering the edges (80 grit)
- remove dust and clean
- laminate layers of fiberglass to build back the surface
- Clean area
- Grind/sand to fair the surface (80 grit works)
- Clean area
- apply at least two coats of epoxy barrier coat (paint)
- after cure, bottom paint

Anytime you lay new glass you will have pin holes in the surface. I don't think it is possible to eliminate them when laminating. I had exactly 1,234,321 pin holes in my boat. The epoxy barrier coats should/might fill them, but you might need to use some poly-based filler (bondo) to catch them all. You'll see them appear after you apply the barrier coat (white in color). Lightly sand and apply another barrier coat after filling. You don't want water getting behind the paint...ever. That's why it's called a barrier coat. Poly works better than epoxy for filling pin holes...they are so small and not a strength issue.

Much obliged guys.

Earley bird, thanks for the wishes!!

Cam, strakes, I missed it.

So, the epoxy BARRIER is different from the epoxy that we’ll do the work with, right?
You all agree that I should finish with that, before the antifouling.
What about any stucco/paste-like material to even out some rough surfaces BEFORE any barrier is applied?
Does it have a name and does it have to be compatible with that epoxy barrier?

Example of Epoxy barrier coat:
https://www.jamestowndistributors.com/p ... m8QAvD_BwE

If you need or want to make the surface smooth, use an epoxy-based fairing compound prior to barrier coat. Example:
https://www.jamestowndistributors.com/p ... detail/860

I had quite a bit of water in my strakes. Over the life of the boat, they were damaged and repaired in many places. The old repairs trapped water inside the strake. I drilled small holes at the transom end of the strakes to let the water drain out, then repaired all.

Thanks Cam.

Hey, they’re not kidding with their prices!

Yannis

If you can, use the Interlux product for the barrier coat and fairing. We used it way back in 1986 and it has held up to this day.

SeaHawk makes an outstanding barrier coat product.

Thanks Rick,

I haven't started the job yet cos Im doing other stuff.
The consecutive covid lockdowns have altered the plans for everybody.
So far, the area has been grinded (or is it ground?) clean and feathered appropriately, but left there.
I’ll put up pics as the work progresses.
Thanks.