Sanding ply before epoxy?

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Buz
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Sanding ply before epoxy?

Post by Buz »

I am just a simple caveman trying to make a nice boat. I do not understand all what I see here. This one thing has been buggin' the heck outa me more than any other!

When whittling a bow or making any other wooden thing that I want to look nice, I would never consider applying a sealing coat before rubbing it smooth with at least 320 or better. Any less than 400 or so would be what I now would call a work-bow finish. Colors in wood pop after sanding. It seems that slathering epoxy on wood that will be left bright should be done after a proper sanding job.

Similarly, I've also seen it suggested to go from primer to paint at 120. Paint fills 120? Bear grease doesn't.


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topwater
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Re: Sanding ply before epoxy?

Post by topwater »

When i sanded the primer for paint on my boat i went to 220 , some guys go to 320 . Do what
the instructions say that comes with the paint and you should be fine.
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Re: Sanding ply before epoxy?

Post by Browndog »

This question is a funny one because it depends on the personal experience, the size of the boat project and amount of obsessive compulsiveness of the boat builder to truly appreciate the question and provide an answer.

Sanding the bare plywood before applying epoxy will theoretically improve the finish. You are correct. If that is the only finish being applied. So if you are making a bright finished dinghy or lightweight boat with only the chine and transom seams being taped and glued you'll see a difference. If the boat is anything bigger, see below:

However, when constructing a boat most of the plywood will then be covered with fiberglass cloth which when wetted out is transparent, but will lay over the now sanded plywood and require some type of surface treatment as well because of its lack of UV resistance, it's weave and inevitable bumps, lumps, wrinkles, loose fibers, dust, insects trapped in the goo, etc. That is why most hulls of any size, using fairly coarse heavy fiberglass cloth requires an additional step called "fairing the hull" to obtain the smoothness of finish that you refer to. :(

Even with multiple fill coats of epoxy and many hours of sanding with progressively finer abrasives, many top coat finishes or primers require a certain amount of "tooth" to adhere mechanically if they are not capable of a chemical bond. Thus you'll often see surface prep done with no finer than 120 or 220 in most cases.

If you want a superfine bright finish over a wood epoxy fiberglass composite sandwich then Varnish is the way to go. After many rounds of sanding and multiple coats of Varnish with obsessive attention to detail during application and good lighting to reveal "holidays" or spots that didn't get covered, you'll have a baby butt smooth and glossy mirror like finish that you'll love and admirers will go Gaga over. And when it inevitably gets scratched, will be fairly easy to touch up with another layer of varnish. :D

The most frustrating part of building your own boat is when it seems to be nearly done, there is a lot of "finish" work that is between you and the end. Most of the finish work takes the form of sanding. And then sanding some more. That is why you hear a lot of discussion about workboat type finish vs. yacht finish. Most of the difference is amount of surface prep and attention to detail before and during the final topcoat/paint application. It is also what causes some projects to take a lot longer than initially estimated, because it is your pride and joy and every imperfection, drip, ripple, etc. must be corrected. It is why some people, refinish their boat after it is done because they weren't satisfied with the amount of "whatever" when it was done. It is also, what sadly causes some projects to be assembled, but never finished.

The truth of the matter is, very few or no one else will notice the little things that nag you about the finish. Unless stored in a museum, even after a modest amount of careful use, your boat will be worn, bird pooped on, spider stained, mineral and black stains from water, and scratched beyond comprehension. Obsessive attention to detail on the bottom, is especially unnecessary if your boat is larger than a car topper, stored on a trailer, stored on a lift, or have its bottom painted and kept in the water.

The bigger the boat project, the more complex the rigging and inevitably more surface area required to be finished, the more time involved, and you can see it only gets worse. It is amazing what slapping a coat of paint can do to improve your mood when it comes to working on a boat.

I look forward on my next boat to the liberal use of non skid paint on the top sides to provide an attractive, durable finish that can be applied without a ton of sanding. Perhaps even spray the whole thing with Rhino liner and be done. Heavy, butt ugly, but indestructibly done. :lol:

Good luck with your boat building projects.

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Re: Sanding ply before epoxy?

Post by topwater »

I did my whole top sides with Monstaliner for those very reasons .
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Re: Sanding ply before epoxy?

Post by cape man »

It seems that slathering epoxy on wood that will be left bright should be done after a proper sanding job.
The stitch and glue method doesn't lend itself well to a total bright wood finish as there are ugly joints that really need either a butt block or biax cloth, and just getting the grain of two different pieces of ply to line up is hard. However, there are numerous places on these boats where a single piece of ply can be sanded, sealed, and bright finished.

If going bright finish, yes you should sand it down and go as fine as you re willing.
The world always seems brighter when you've just made something that wasn't there before - Neil Gaiman

Buz
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Re: Sanding ply before epoxy?

Post by Buz »

Good point. I guess I should have made my intentions more clear. Once I realized fairing filler was going to be visible I decided to make only trim pieces and seat tops bright rather than the whole inside or some such. Paying extra attention to that bright wood makes sense, I'd just not seen it mentioned.

I've gone the varnish route with a few bows. I learned to do something different because I use them. Being familiar with it is why I'd prefer to sand the ply, epoxy it, then sand the epoxy smooth before applying a UV clearcoat. I plan that based on my assumption that a marine-type clearcoat is more durable than varnish.

The more I think about it the more I think leaving wood bright dooms one to having to keep close watch for scratches and constant refinishing to keep UV from the epoxy.

Monstaliner- Thanks for that tip. Does it have the grip it looks to have? I'm building an FB11 first. A durable finish where parts are going to rub while stacked is something I'm looking for. I plan to glass those spots, too.

I sure do appreciate all the thoughts and the time you took to put them down for me.
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Re: Sanding ply before epoxy?

Post by Cracker Larry »

I've done a lot of bright finish trim work. With teak, mahogany, cypress and ipe. I sand them all to 220, then 2 coats of epoxy. Then sand them again to 320 usually, 600 if I really want it nice. Then 2 coats of Awlgrip or EMC clear coat. Sanded between coats.

This GF18 has a mahogany ply rub rail. I covered it with 6 ounce woven cloth and it became almost invisible. Again using UV resisting clear coat over the epoxy. If you look close in certain light you can see the weave, but not from 3' away.

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Then we used ipe trim strips for good gunwale protection, and for a lot of other trim work. Sanded, epoxied, sanded, clear coated..

Image

The deck and steering pod, and some other stuff is cypress. All sanded, epoxy coated, sanded, clear coated...
Last edited by Cracker Larry on Mon Aug 22, 2016 12:08 pm, edited 1 time in total.
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Re: Sanding ply before epoxy?

Post by Jeff »

Beautiful work CR!! Jeff

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Cracker Larry
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Re: Sanding ply before epoxy?

Post by Cracker Larry »

Thanks Jeff. This ain't my first rodeo :lol:

Image

Image

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Buz
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Re: Sanding ply before epoxy?

Post by Buz »

Holey smokes!!

Seeing what you did there already had me thinking of making a similar sole in my rowboat to provide a flat (and dry) spot down the middle.

Does that clearcoat provide a longlasting durable finish over the ipe that doesn't need the "maintenance" that a varnish will? I know ipe will quite take a lick before denting. I suppose glassing the bright (Meranti) seat tops would be a good idea.

(Ah kin make a good bow outa ipe and bamboo.)
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