For the 10,000th time... Polyester + plywood.

Then do not use wood. Use closed cell polyethilene foam instead as a former and glass over it. Best way to do it is to vacuum form. The fiberglass has to provide the strength. The PE foam is a marine foam and would not absorb any water. If you sandwitch the foam it will provide stiffness.

No kiddin, the polyester does not work well with wood, epoxy does.

 

just a thought !

On my boat i made some wood furnature 15mm thick 7 ply gave it 2 coats of poly resin thinned with styrene ,when gelled sanded and then a brush coat of celcoat mixed with 20 resin and wax .
I applyed the coat quite thick to be able to sand the brush marks out , then a sprayed coat of gel coat but this time thinned with acetone with wax !!
it was good enough to simply leave and use as it was . Been in use for 4 years and looks better now than when i did it first .

 

Here's how I join the polyester fiberglass parts.

The pieces there loaded with 100kg either side on top does not pry the pieces apart, standing on a support in the center only. The sealant area is 60mm wide.

I don't have a part number for the dow corning, which is what you see between the fiberglass pieces. They also join car panels (ie Volvo) using these. It's not silicone - do not use silicone !

 

Hi Tunnels,

It will probably last if it's not being worked and if it stays dry. I'm an absolute polyester fan, (unlike Richard) and I use only polyester, but I have polyester glassed marine ply before and had to take it out about two years later - it was the deck of a boat. I won't use wood again though.

 

Everyone knows my opinion about polyester resin on wood, but it can be used.

Heat isn't really going to get you much, because it's on plywood most of the time, but slight dilution with the first coat is a good idea and effective too.

Film thickness and encapsulation are the real tricks to polyester on wood. Sufficient film thickness does a fair job, good enough that manufactures still use the same techniques that permit me a reasonable repair business on transoms, soles, stringers, etc.

Some manufactures get better resin coverage and use better techniques, which keep their boats out on my driveway longer.

No, it's not going to be nearly as good as a part embalmed in epoxy, but it can work for some time. Generally, I'm seeing boats from the early 90's, which suggests the manufacture's are capable of getting about 15 years of service from their products, before the worst of them need structural repairs. This is perfectly adequate for a project with an intended life span of this or less.

Sometimes relevance is an important consideration, particularly in the epoxy/polyester debate.

 

what's the difference in cost of a gal of epoxy and a gal of polyester?

 

Warrd, in my world the materials are typically 13% of the cost of repairs. If the resin cost is $25gallon (poly), 50/gallon(good vinyl), 100/gallon(epoxy), then who cares what the resin costs? Nobody on this forum is going to debate the relative merits, epoxy is king!
However....if you want to make some cocktail barges, car parts or lots of stuff that comes out of molds........ nothing wrong with well engineered polyster and glass fibre. Doesn't it ultimately come down to the workmanship and knowlege put into the product?

 

All retail prices - general purpose polyester resin (good quality boatyard resin) runs about $30 a gallon. Good laminating resin $60 to as much as $100 a gallon.

MAS epoxy is about $105 (slow hardener) per gallon. West System epoxy is about $110 (206 hardener). System Three epoxy is about $90 a gallon for it regular resin, $110 for Silver Tip. Marinepoxy is $62 per gallon. Progressive epoxy about $60 per gallon.

Naturally, larger quantities bring lower pricing as do wholesale options. With the advent of the "less refined" epoxies on the market (Marinepoxy, Progressive, etc.), the excuses for not using it because of the cost, are just not true any more. I've used both Progressive and Marinepoxy and they perform quite well.

 

It's a rage this mania of polyester with plywood. Simply it doesn't work. Polyester is not waterproof and does not glue even on itself after curing. It's made for lamination and not more.

You're right PAR, polyester will last 15 years at best if the boat navigates only 100 hours/year. On work boats with 1500 hours/year, let say 2 years if the Holy Virgin of Gualadupe intercedes for you, after you'll begin to have structural problems. But that will need a lot of resin and glass fiber with tons of gooey, plus a lot of work, while breathing chemicals like styrene, acetone, cobalt and peroxyde... Heavy and expensive at final.

Epoxy needs less work, you'll use less so finally it won't be more expensive than polyester. Raka has a resin the 127 that works well for 62 USD a gallon (less if you buy a kit of 10 gallons for example). It does not stink...and it works. Epoxy/plywood is known and proved since 40 years at least.

Practice of USA shipyards of low cost plywood with orthophtalic polyester is not a reference of good engineering...

The epoxy resins at 60 USD gallon are not less refined, simply they are not overpricing the name's brand. Practically all the marine epoxy are formulated on the same basis, most of the time a Derakane, using the same reactive solvent to adjust the viscosity plus a few additives. Nothing mysterious, it's basic chemistry.

 

Gentlemen... Don't you think it is interesting that this thread is on page 2 and chugging right along towards 3 and maybe 4...and the OP hasn't so much as peeped in since the first post? Can you say a vigorous stirring of the fecal matter? Someone is tapping on your knees.
Whoosh...Whizzzzzzzzzzzzz....Plop....Reel, Reel, Reel...Tug...Yank...Gotcha!

 

Hi Ilan

I think you may have misread Par's post. Polyester fiberglass boats, if done correctly will last as well as any out there. Just do not use wood. There are many ways to overcome the problem.

Polyester is really a good media, though is not always applied correctly. If you expect it to be strong and last, then do the glassing in one go. On parts you do not have a choice, ie when retrofitting something, glue it in place instead of glassing.

Btw, dry epoxy with a new layup also delaminates. Maybe a bit less easy and it may be slightly more flex tolerant, but it still delaminates. Can pull it off same as I can polyester. This is the main reason I use a sealant to join parts now. To remove again if required you can saw it off with a braid line.

If I'm not mistaken we pay about 5x more for epoxy here.

 

It's not a troll, it's more comparable to a feeding frenzy. The OP is only looking to replace a few fairly inconsequential supports and got what info he asked for by post 3, then some drift, then the thread is pretty much ended by post 7. Then the foodfight starts. The project is a thrifty, noncritical application using materials at hand is what I figure. Why have a 100% poly boat and then go out of your way to have a 1% epoxy boat? Of all the supports he has in the boat, the odds are the ones he just replaced with poly will outlast all the others, especially since he is using marine ply and is taking the trouble to saturate the rot prone areas much better than most mfgrs ever do. What would be nice is if posters such as AMCr (which name points out using epoxy here would only be putting lipstick on a pig) would come back with a "thank you" or better yet, throw a party for us with free beer, live music and strippers.

Myself, I'm glad the thread kept going. I didn't realize the price of epoxy vs poly was as close as it is and if I ever do a big project where it matters, or a new build, epoxy will be the resin.

 

Not for repairs Fanie. For a newbuilt ok, but as mentioned a thousand times here, poly does not stick well on anything but uncured poly.

 

You think so ?

 

Oh sweet God, I dated her years ago. She's appears to have held up well over the decades . . .