Sanding Epoxy inside hull

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Francis
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Sanding Epoxy inside hull

Post by Francis » Wed Aug 01, 2018 10:12 am

I was wondering how far I must sand the inside epoxy layer (I have the intention to put 2 layers) before I want varnish the inside. Is it a problem if you see the weave of the fiberglass after sanding. I know, not sanding true the weave... I want to keep the weight of my first racing canoe as light as possible.

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Patricks Dad
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Re: Sanding Epoxy inside hull

Post by Patricks Dad » Wed Aug 01, 2018 4:12 pm

I'm a bit confused. Are you asking how much to sand the first layer of epoxy (with an intention of using two layers of epoxy (rather than 2 layers of glass?)?

I'm assuming you mean two coats of epoxy (2 layers of glass would add weight you don't want)...

You just need to scuff the surface to allow for a mechanical bond. You don't want to hit the glass or you will still see it after you add the 2nd coat. Don't be tempted to sand into the weave to reduce weight. If you squeegeed well on the glassing job there shouldn't be anything there to take off.
Randy Pfeifer
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Cruiser
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Re: Sanding Epoxy inside hull

Post by Cruiser » Wed Aug 01, 2018 8:40 pm

It is common to only use 2 coats of epoxy on the inside to leave some texture to the finish, to make it less slippery. If you haven't done it already, you just roll on 2 coats of epoxy ... instead of 3, which is the norm on the outside. Then a light scuffing is used before varnish.

If you have already done the initial coat on the inside and have let it harden, and are now asking how to prepare it for a 2cd coat of epoxy ... I personally would rethink that strategy. You mentioned 2 items "race boat" and "weight" ... if you have let the initial coat cure, there is little purpose in adding another coat of epoxy, the strength is in the first coat, the others are "fillers" to make it look smoother. I would think about a light scuffing/cleanup and moving on to varnish.

I may have misread the post, but that would be my opinion.

Brian

Francis
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Re: Sanding Epoxy inside hull

Post by Francis » Thu Aug 02, 2018 7:06 am

Thanks for the replies. For the moment I just started with sanding the inside. My idea was to put a light fiberglass (80 gr. sq/m), wet it out with epoxy, put a second piece of fiberglass (same weight) in the area from feet to behind seat, epoxy for this part and when still sticky a filler layer for the total inside with a roller. Let it dry and a light sanding (120 grid wet) and 2 layers Epifanes with a foam brush. I don't have inner or outer stems, so is it better to fill the inside bow and stern with a thick fillet of epoxy?

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Cruiser
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Re: Sanding Epoxy inside hull

Post by Cruiser » Thu Aug 02, 2018 8:06 am

I think it might be best to wetout both layers at the same time, 2.8 oz cloth is very light and soaking in will not be an issue.
Rather than a fillet, I think I would try and use a strip of bias cut cloth on the inside of the bow and stern ends. The cloth will give structural strength, using a lot less epoxy. I know the fillet is usually used on stemless, but epoxy weighs in at 90-100 #/ft3, so a thick fillet is going to weigh in at significantly more than if you had used stems ... seems to defeat the purpose IMO.


Brian

Francis
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Re: Sanding Epoxy inside hull

Post by Francis » Thu Aug 02, 2018 9:49 am

Thanks Brian. For the extra strength I can use a 4oz bias in bow and stern. When I put the second layer fiberglass (from footbar to seat), is it a good idea to lay them in an angle (45°) to give it 'extra' strength?
Is there a good trick to avoid the feathering of the glass when you apply epoxy. Some suggest to lay a kind of plastic wrap above the epoxy when they use a roller, but I was wondering if the heat of the epoxy isn't a problem...

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Jim Dodd
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Re: Sanding Epoxy inside hull

Post by Jim Dodd » Thu Aug 02, 2018 12:39 pm

Welcome to the site Francis !
Wetting two layers at a time is fine, unless it is tightly woven cloth.
Make up a test layup first, of about one square foot !.
I ran into trouble wetting out just one layer tight woven cloth.

Laying the cloth on the bias is supposed to increase strength. But by how much ? In real life. I doubt you can tell a difference.

Jim
Keep your paddle wet and your seat dry!

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Cruiser
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Re: Sanding Epoxy inside hull

Post by Cruiser » Fri Aug 03, 2018 7:33 am

Thanks for weighing in Jim, you are about the only builder I "know" who has actually used S glass, since Francis is using 2.8 oz material, do you think the wetout of 2 layers would still be an issue? Is wetout of S glass generic or weight related?

Also, you build stemless and Francis was going to put a thick epoxy fillet on the inside (where bow/stern meet) for strength, the bias cloth strip was suggested as an alternative, since a bias cut would be more "flexible" fitting the contours there. Using the thick epoxy fillet would seem to negate any weight saving going stemless would provide and using the cloth here should provide more than sufficient strength in this area. Again, your thoughts here would be valuable.


Brian

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Patricks Dad
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Re: Sanding Epoxy inside hull

Post by Patricks Dad » Fri Aug 03, 2018 8:06 am

I found S-Glass to wet out just about as easily as E-Glass. I'm more accustomed to 4 oz s-glass however. I would say that it wets out easier than 6 oz E-glass (but there's 2 variables for you). It probably also depends on the viscosity of the epoxy you are using (I was glassing with West System epoxy on a pretty hot day so that probably made it go more smoothly too - 3 variables).

But I would not hesitate to wet out 2 layers of 3 oz S-glass just because it's S-glass...
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Jim Dodd
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Re: Sanding Epoxy inside hull

Post by Jim Dodd » Fri Aug 03, 2018 9:33 am

It's the weave of the cloth, more so than whether it's S or E-glass, that can make wetting out difficult ! It's air entrapment.
Yes. S-glass wets out fine in regular weave.

With stemless, it's quite easy to over lap cloth in the stems. That is really all that is needed. Fillets are hard to keep from sagging in a vertical fill.
Generally I get by overlapping, but if I really wanted to beef up the stems, an inner bias strip of about 4" wide goes in over the top of the overlap.. I do this just after, I have the inner layer whetted out. And yes this would be lighter than stems.

It's so much easier than dealing with inner and outer stems. I know the stems are "Traditional", but it's really a throw back to when boats had to be built this way.

Jim
Keep your paddle wet and your seat dry!

Francis
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Re: Sanding Epoxy inside hull

Post by Francis » Fri Aug 03, 2018 10:30 am

We know what to do. Thanks for all these wise words!

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