Using Fiberglass Resin as Primer and Sealer


I have an old plywood rowboat which periodically is repaired with fiberglass cloth. This has worked very welll- the boat is now about 70 years old! I just did some minor repair work including bottom painting and fiberglassing. The bottom is still quite rough as over the years there have been many layers of paint which has peeled off unevenly. Even heavy sanding won't fix it completely. For years it has seemed to me that painting the entire bottom with fiberglass resin (no cloth) would make an extremely durable and water-resistant primer, to be painted over with polyurethane marine paint. Is there any reason this shouldn't work really well? Thanks in advance! Frank
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frank1492 wrote:

That is historic and should be preserved! I recall plywood being used for sailing dinghies in the late 40s here in the UK but not before WWII. I'd be loath to do what you are proposing.
I sanded down many dinghies of the 50s era and when not wishing to explore too deeply used stopper to smooth off the surface. I could feather that down so smooth that I could get a fine finish to the paintwork.
I could easily be wrong but wonder whether you might be creating a layer that would be pierced and trap water?
The nearest that I could find to stopper these days is Sikkens.
http://www.sikkens.co.uk/en/Products/Alphabetical/GupaWoodfiller.htm
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Clot: This actually was a custom-made sailing dinghy. My father had it on his cabin cruiser when they went to the 1939 World's Fair in NY. It has a lot of freeboard, very practical in the chop here on Cape Cod, MA, US. It was punctured in a couple of places during a hurricane, but my father patched it. I too am concerned about trapping water, which may be a good reason not to do this. However, with fiberglass patches applied liberally over the years, particularly where the sides meet the bottom, and still no serious rot anywhere, I am inclined to be somewhat less concerned. However, I will consider your thoughts carefully. Please note that I do not have the time or inclination to do any kind of fine finish work on this boat. My goal is simply to keep it leak-free and in one piece. Thanks very much for your thoughts. Frank
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the resin bond will only be as good as the paint bond underneath the resin. if the resin is applied to bare wood,then it will bond to the porous wood,much stronger than to paint.
I'd use stripper and sanding to get down to bare wood before applying resin.
www.systemthree has the Epoxy Book,a great resource about using epoxy resins for fiberglassing.It's a free download.
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Jim Yanik
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Hi Jim- Thanks for your suggestion. I really don't have the time or the inclination to go down to the bare wood. I would have to take a chance on the paint as a base. (Would check that carefully before applying the resin, though resin over the paint historically seems to give a very good bond.) Will look at System Three. Thanks very much. Frank

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After being patched with fibreglass, no wood boat maintains its antique value (if any.) Main considerations seem to be: 1. Total weight of a boat with resin skin 2. Possible effects of the resin on the adhesive holding together the wood plies (and no plywood glue made 70 years ago was expected to last long in a wet environment.) So long as the resin skin were perfect, you might be OK. Any scrape or imperfection might fatally weaken the whole.
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Don Phillipson
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frank1492 wrote:

Yes.
1. Cured polyester resin is quite brittle. Fine with fiberglass cloth, not good by itself.
I speak from experience...I once did all the considerable bright work on a 42' yacht with polyester resin; thinned it, brushed several coats, wet sanded, polished with rubbing compound. It looked great. For a while. Until it started cracking. Lopts of work to remove and replace with varnish.
2. The solvents in the resin *may* affect the paint to which it is applied. OTOH - given the prevalent use of Bondo - maybe not.
3. Adhesion would depend upon the condition of the paint to which it is applied. Since there are "many layers of paint which has peeled off unevenly" I sure wouldn't trust the adhesion.
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Best would be taken down to wood. You are disinclined to do that.
Next best would probably be to chemically strip paint, fair up clean surface with resin and Cab-o-Sil or other filler, apply a thin, overall layer of glass cloth and resin. then paint. That's a bunch of work too.
Given your constraints of time and interest, I think your best bet is to simply paint; if necessary, use a conventional primer first.
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dadiOH
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Thank you both for your comments. As for antique value, really don't care. As for fatal mistakes, fiberglass cloth has been rather crudely slapped on this boat for about 30 years with no major rot occurring. dadiOH's reply is really what I was looking for. I assumed there would be an issue of flexibility with just resin, although where the resin overshoots the edge of the cloth, I have never seen much evidence of cracking, even in completely unsanded areas. (How *do* you remove it once it hardens?) But then it has been applied over polyester paint- perhaps that makes a difference. I think your final advice to "simply paint" is a good one. I have painted with Interlux, but have applied polyester resin (no cloth) to a few ragged areas. Will see what happens. Again thanks to you both. Frank
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frank1492 wrote:

I always used a disk sander with about a #16 grit disk. Fire works too, it burns really well :)

Interlux makes good stuff.
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dadiOH
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west is rather pricey when it comes to epoxy.
www.uscomposites.com has much better prices. You can tint a thin epoxy and "paint" with it. It will last a loooong time.
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On Fri, 25 Sep 2009 10:22:41 -0700 (PDT), jamesgangnc

Epoxy is generally very vulnerable to deterioration from UV exposure.
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