Re-coring question
 

Do I need to sand/grind this down more to remove all the unwetted strands from the original layup? I removed the original core but remaining is very uneven.

http://i1326.photobucket.com/albums/...pswzgd93zf.jpg

I would grind all those dry strands and get down to solid glass. Then trowel on some thickened epoxy or polyester then the core.

I recored all the hatches on my 1975 18 and on my friends 1972 23.
Trying to remove all the "dry strands" is almost impossible without grinding the existing glass to nothing. It gets so thin so fast.
The glass on my 18 was a little thicker than the 23. The hatches on the 23 ground away to nothing so fast, I burned through in a couple of spots.:o

What I did was remove as much loose stuff as possible with, I think a 60 grit on a grinder.
Got it mostly flat, then wetted it all out with un-thickened vinylester.
Let that kick for a little while, and while it was still green, backbuttered and bedded down my core with vinylester hull and deck putty.
I put some weights on top (batteries I think) and troweled and filleted the edges where it squeezed out and let it kick again for a little while.
While still green but not wet, I wetted out templated and pre-cut 1708 and laid that over the top smooshing it into the corners and edges and getting all the air out.
Then let it all kick.
Worked out great.
Just remember to bevel or fillet the edges of your core, as the 1708 will not wrap around a 90 degree corner. And when you get to the "add weight" part of it, put the hatch on a nice hard FLAT floor, garage, basement, whatever.
I did one hatch on my 18 that I left up on a wonky workbench and it got a weird little twist in it. :mad:

Good info. Thanks Zak. That's what I'll do.

I have a few pics of my recore.

I (think I ) didn't use thickened VE resin. I used un thickened. I didn't grind much. I did sandblast and put a layer of mat between the skin and foam.

http://www.classicseacraft.com/commu...pictureid=1449

Starting here, halfway down has more details than I can remember. Other than the backache.

http://www.classicseacraft.com/commu...t=25405&page=7

in this situation:

I attempt to fair the surface,before coring it.this will assure you have an even surface,with full core contact...

coring to a dimpled/rough surface - this can and will create a few low/high spots in the fiberglass skin

Doh!
I did this too. Totally forgot about it and didnt put it in my description above.
I templated and wet out a piece of 1708 with unthickened VE, and put that right into the lid. Let that kick for a bit, then installed the core with thickened VE on top of it, then went to the last piece of 1708.
It's true the extra layer of glass helps smooth out the internal surface of the original glass while also adding a little strength.

Thanks for reminding me! I knew I had forgotten something...

that will add tons of strength,also weight...try not to use too many 1708's.. great fiber and strength.but do not need too many layers....try this,.grind out the dry weaves,the skin cant be that thin,BUT,be carefull..put thickened resin(your choice)on the skin side and the core side, and put together with weight to force the excess out.clean up the edges and when dry put 1 layer 1708..use a hard roller to even it out.If you have access to a vacuum system, its a different process.It will all work out in the end !

So this is what I've done so far. Removed wood core, cleaned with acetone, mixed some poly resin and applied to the whole underside to saturate the un wetted strands. When dry I roughed it up with 60 grit, wiped with acetone again and applied system three fairing compound to make it as even as possible. Tomorrow I will sand that down and apply more fairing compound as necessary and sand to get it damn close.

After that I will cut to fit the Nida core leaving a one inch perimeter all the way around. Then I will cut a piece of mat wet it out and put it on the under side. Before it fully kicks I'll put down some hull and deck putty then the Nida core with weight applied. Once cured I'll cover the core with one layer of mat followed by one layer of woven since I don't have any 1708 on hand. Does that sound sufficient?

It sounds like your using epoxy products (System Three) and if so it might not be compatible with straight mat that has the binder. The epoxy doesn't dissolve the binder well, if it's a sewn mat or biply then no problem.
Rod

I used system three fairing compound cause I had it left over from anothe project. All resin will be poly because that is what I have used for everything and have a gallon left over. However you got me thinking now. Poly won't adhere to epoxy products well from what I've heard but epoxy will adhere to poly. Can anyone verify that or is it one of those old wives tales. It's just a hatch so I think I may still stick with the plan of using poly resin over the system three fairing and cross my fingers...

Whenever I've tried poly over epoxy I wound up with curing issues. It seems to react better if the epoxy has cured for a longer time like 30+ days and it has been ground to degloss it. If you want to push the envelope (and why not lol) I would try a test piece.
Rod

Ok change of plan, gonna buy a 3 quart unit of system 3 epoxy. Also gonna get two yards by 50" wide of 17 ounce double bias cloth to lay down over my fairing compound before the core then lay another layer on top. This gonna be strong enough? All will be purchased from Mertons in MA.

Personally, I think you have to go all epoxy now, or grind out the fairing compound.

I'd go polyester, personally, as it wets the mat. And I'd just clean it up with a 36 grit belt sander. I'd skip the hull and deck putty, and pre-wet the nidacore skin. The nidacore I have took resin well. I used it with VE and 1708 for my "Whaler bench seat/tank cozy" project.

You do want thickness under the nidacore/ for the topside of the hatch for penetration resistance. Much less is needed on the underside.

With respect to mat- plain old biax may lay flatter, so you might want to skip that on the topside of your laminate (the bottom of the hatch). It gave me fits. ( I made note of this)

I would also put tape or biax in the trough section of the layup- for better screw and screwhead holding.

I in-lined rather than linked a pic of the finished product (with PVA and bits of glass still on top. I put peel ply on top, then paper towels, edit: THEN 4 MIL POLYETHYLENE SHEET, then plywood, then car batteries. That was almost too much and started to dry out the laminate, but that's because it didn't kick for nearly a day. That approach should be fine for a normal layup.

And no, the H80 core isn't centered, but it works.

Yes. And if I were to go epoxy, that's about what I'd do.

That epoxy layup will certainly work strength wise but it'll pricier than a polyester lay up for sure. I agree with Fishstretcher and would use polyester for this job.
Rod

I've never used epoxy before so this will be a good piece to tactics on despite it being about twice as much, at least I'll learn something. Thank you very much!!

I will throw this out there because I always do.

Mertons sells a nice vinylester epoxy. It has equivalent or better mechanical properties to a "conventional"epoxy. It seems even better than some. But the cure system is like polyester. MEKP and cobalt. And it wets out mat and you can thin it with styrene monomer. It is also said to stick to "conventional" epoxies and polyester. It certainly has for me. And some sailboat guys use it for just that. The formulation he sells is very thin- good for infusion. It is also quite reactive- it will gel in sunlight withou MEKP.

People think it is like polyester because of the styrene smell but vinylester is the short name. Generally it is listed as an epoxy- vinylester epoxy.

So that's an option

I have posted extensively on this before- with a bunch of data. If you are interested I can dig it up.

Be careful with conventional epoxy- you can develop an allergy to the hardener and never be able to use it again. So use the gloves. A few layers so you don't muck up the wrists when you peel off gloves.

General rule of thumb re: epoxy and other resins:

Epoxy over everything, nothing over epoxy.

;)

Perfect:D

Can Nida core be cut with regular tools, ie table saw and circular saw?

Also Fishstretcher had a very good point, work very clean with this stuff. Back in the '70s & '80s we were practically taking baths in epoxy and as a result I became sensitized to the hardner. We were using a lot of System 3 & Chem Tech back then and they didn't have many options for hardners. I can get away with using it on small repairs but anything over about 1 sq ft and I can't get near it.

I have a couple sheets in the shop and you could certainly cut it with a jig saw, skil saw or table saw. It's really light and easy to handle.
Rod

If it is pre skinned, it cuts with a saw. I think you got raw core from LBI. You can almost cut it with a utility knife. I bevel cut mine with an 18v panel saw.

Everything I read on boat building shows a VE layer between regular epoxy sailboat hulls and polyester gel coat. And that is also what Duratec sells- an intermediate vinyl ester epoxy layer.

Maybe it should be VE on everything, epoxy on epoxy and polyester on polyester?

Of course rules of thumb are made to be broken. By Murphy's hammer.

I did get the raw core. i'll try a small piece with the table saw and see how it does. I will be putting a 45 on every cut edge for glassing reasons and if the honeycomb is exposed by the cut, i'll fix with thickened epoxy for a nice finish. Thank you all.

I'll be sure the wear appropriate PPE before begining. Thank you.

That's just what I was always told. I always considered it a "general rule of thumb" that applies to general fiberglassing, basic boatyard repairs, shadetree stuff.
All thumbs are out the window when you get into more technical glassing.
You could go down the rabbit-hole real quick just browsing around at the Composites One website.
:eek:

Well, I had a couple university classes in composites 20 years ago, and can't remember much other than the math for stress analysis for multi layer uni axial layups of carbon in epoxy was hard if you do it by hand.

With respect to resins? We built a lot of VE structures at school. The lead fabricator (school employee) had developed an epoxy sensitivity.


But I don't own a boatyard. All I can do is interpret data sheets (I have done most of the ASTM tests at univeristy years ago). But that isn't the same as building boats for 20 years.

But my first scratch built monocoque fiberglass (R/C) car was at age 14 :D.
Then a 15 year hiatus or so.

But I am a weekend warrior, mostly. I only ever got paid to work on aluminum "Navy" boats, and then only briefly.


So like any advice on the interwebs- you get what you pay for.

I used all epoxy and it's solid as a rock now. Thanks for the help