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Gelcoat or paint?

Jun 25, 2004
283
Hunter 306 Pasadena MD
What would you use to finish raw fiberglass in the bilge: gelcoat or paint? And if gelcoat, what particular product would you recommend?

This is for polyester resin (Evercoat Kitty Hair) used to bond thru-hull backing plates:

head_pickup.jpeg
 
Jan 11, 2014
7,746
Sabre 362 113 Fair Haven, NY
If this was my boat, I'd sand off the sharp pokey bits, and then apply a thick coat of gelcoat. Gelcoat will fill in some of the dips and valleys better than paint making it easier to keep clean.
 
Mar 26, 2011
2,916
Corsair F-24 MK I Deale, MD
I'm sorry if this seems negative. Some of these comments are for the next guy that tries this or for your next attempt. This one will be OK.

  • Polyester resin is not going to bond well to a PVC Groco backing plate. It will probably crack off in time. Use ether epoxy or, better yet, polyurethane sealant, which would bond better. The best choice would be a FRP backing plate, which would bond better. There is no point in putting bolts in something that is not a part of the hull, IMO. If you are not going to well-bond a FRP plate, you are probably better off with traditional through bolts.
  • You don't bond on top of paint. You grind back to FRP.
  • I hope you sanded and flamed the PVC backing plate first. PVC does not bond very well.
  • Kitty hair is used when you need flex strength. Thickening resin with something fine in texture, like fumed silica, would do a much neater job.
  • The bilge is painted, meaning gelcoat won't stick well. That's probably Interlux Bilge Coat or something similar, so I would go with that.
  • Before you paint it you will need to fair (smooth) the surface by coating it with resin thicken with something smooth.
  • You won't be able to twist the seacock off for service. I guess you know this.
It should look more like this. And to repeat, I don't see the logic (other than cost) in making backing plates from PVC, which does not bond very well, and then going to the trouble of puting studs in them and trying to bond it. Makes no sense.
 
Sep 25, 2008
6,271
Alden 50 Sarasota, Florida
Polyester resin (e.g., Bondo by any name) should not be used underwater. It isnt impervious to moisture and will swell and separate. Replace it with the appropriate resin glass material after which you can hide it with whatever you prefer. Neither gelcoat or paint are impervious.
 
Jun 25, 2004
283
Hunter 306 Pasadena MD
Well, I'm certainly no fiberglass expert. I do feel very comfortable with the backing plates I just glassed to the hull. I did a moderate amount of research, and decided to use Evercoat Kitty Hair with the Groco backing plates. A few notes:

1) The Mainesail article Seacock Backing Plates - Marine How To says "Whether epoxy, polyester or vinylester all should be plenty strong for this application." although he recommends using backing plates made from "GPO-3 polyester/fiberglass or G-10 epoxy/fiberglass".

2) In his other article Replacing Thru-Hulls and Seacocks - Marine How To, he uses Kitty Hair, the same product I used. So maybe this is a horrible mistake (using a polyester resin fiberglass where it may get wet), but at least I took expert advice in using it.

3) In the boats.com article about basic boat building practices, Boat Building: Basic Construction of Resin, Fiberglass, and Cores - boats.com, they say

"There are three types of resins: polyester, vinylester and epoxy. ...Polyester: This is the resin most commonly used for boatbuilding today, and most boat owners are familiar with it. It is inexpensive and generally all-purpose. It has low stretch (elongation) properties so it is not used on modern high-performance boats, but it is perfectly adequate for most boats. The most common polyester is an orthophthalic base, but newer isophthalic based polyesters are gaining in popularity. The isophthalics are more resistant to water and chemicals, are more abrasion resistant, and have higher impact and fatigue (flex) performance. Most modern gel coat finishes are made with isophthalic resins."

4) I did worry about whether this particular product (Kitty Hair) would adhere to the Groco plates, but some research convinced me it would probably be fine. These plates bear no resemblance in texture to a smooth PVC pipe: they have a very rough surface and they're being bedded with a filet around the edge. They are also being firmly secured between the thru-hull and the flanged seacock. It's not like I'm attaching a pad eye to the backing block and trying to pull it off: it will be under constant compression in this application. So far (prior to securing the fittings) they seem extremely well adhered. I assume that using gelcoat to finish them (which adheres better to polyester resin than it does to epoxy resin) will help with the waterproofing.

5) As for bolting the flanged seacock to the backing plate, this is recommended as an option by both Groco and Mainesail (in the first article on backing plates: "Many readers have asked me how to install stronger, flanged seacocks without also drilling extra holes in your hull for the bolts that hold the flange. It can be done, as you will see below." I certainly understand that this isn't as strong as bolting through the hull: the main point of it here is to keep the flanged seacock from rotating. And the main function of the backing plate is to distribute load. It's a product made by one of the best fitting manufacturers out there, and I feel comfortable using it.
 
Apr 8, 2010
1,606
Ericson Yachts Olson 34 28400 Portland OR
FWIW... the factory painted on white gel coat in major parts of our bilge, so when I made some changes, I also used gel coat. I mixed in some surfacing wax, catalyzed it, and just painted it on. Lots of smoothing up my work area first with 80 grit. Not a mirror shine, but a real smooth surface that retards the start and growth of mold.
:)
 
Last edited:
Jun 25, 2004
283
Hunter 306 Pasadena MD
FWIW... the factory painted on white gel coat in major parts of our bilge, so when I made some changes, I also used gel coat. I mixed in some surfacing wax, catalyzed it, and just painted it on. Lots of smoothing up my work first with 80 grit first. Not a mirror shine, but a real smooth surface that retards the start and growth of mold.
:)
Thanks for that. I believe the areas of the bilge in which I installed the backing plates are also gelcoated, so I'm leaning towards using that and extending it in a rather broader area. When I glassed in the backing plates, I roughed up the gelcoat with a brass wheel on a drill, as well as with sandpaper, and finished with solvent. I didn't sand down to the underlying fiberglass first, but hopefully gave the new fiberglass something to grab onto.

So you used gel coat, and then separately finished with surfacing wax, right? I've seen gelcoat products which require this (or mold release, they call it), and also one (by Evercoat) which is supposed to be a 1-step gelcoat. Any opinions about the 1-step vs. 2-step products?
 
Jan 11, 2014
7,746
Sabre 362 113 Fair Haven, NY
Gelcoat is a polyester resin and it will not cure if it exposed to air. Laminating polyester resin does not have wax in and will not fully cure until it is protected from air. This is a good thing, because it allows chemical bonding between the laminates. Chemical bonding is stronger than an adhesive bond. Polyester resin has poor adhesive characteristics compared to epoxy.

Gelcoat is sold with wax and without wax. When wax is added to gelcoat, the wax floats to the surface of the gelcoat and seals it so air can not reach the gelcoat and it will fully cure. Another option is to use gelcoat without wax and when there is enough gelcoat built up, it can then be sealed with PVA or a spray wax.
 
Apr 8, 2010
1,606
Ericson Yachts Olson 34 28400 Portland OR
Thanks for that. I believe the areas of the bilge in which I installed the backing plates are also gelcoated, so I'm leaning towards using that and extending it in a rather broader area. When I glassed in the backing plates, I roughed up the gelcoat with a brass wheel on a drill, as well as with sandpaper, and finished with solvent. I didn't sand down to the underlying fiberglass first, but hopefully gave the new fiberglass something to grab onto.

So you used gel coat, and then separately finished with surfacing wax, right? I've seen gelcoat products which require this (or mold release, they call it), and also one (by Evercoat) which is supposed to be a 1-step gelcoat. Any opinions about the 1-step vs. 2-step products?
I was able to pick up some base white gel coat from a local boat builder, and also a small bottle of liquid surfacing wax. I would guess that you can find all this on the www, also. Do a final prep with acetone, also.