Fiberglass Visibility Question

Hey All! 

I'm new to fiberglassing and just laid down the sheet in the cockpit area of a Wood Duck 12. I've got 3 coats of epoxy on it now and it is pretty smooth but I'm very displeased with the cloth weave showing underneath. I read everything I could on what could cause this but I'm worried about proceeding to the exterior glass work until I can determine whether this is my technique or perhaps the cloth I bought (I didn't get it from CLC but it is 4oz "plain weave"). I've attached a link to a photo so you can see what I mean.

I thought maybe I overworked the epoxy so I tried a test piece on another piece of wood and it looked slightly better but still no where near as good as what I'd find acceptable for an eventual bright finish. The wetted out fiberglass tape over the fillets looks pretty decent, and I did the exact same thing for both so I'm thinking maybe it's a cloth issue. It's like it is staying shimmery depending on the angle you view it. 

Thanks to anyone for any advice at all! 



12 replies:

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RE: Fiberglass Visibility Question

Doesn't look that bad in your image.

Three coats'll still have texture, the high spots need to be sanded until they're at the same level as the filled low spots. Try not to sand into the glass fibers though but if you do, and not too far, another coat of epoxy's the answer.

Once you've carefully sanded that surface, wipe it down with denatured alcohol & you'll get a good idea what another coat of epoxy will do, minus the high gloss.

Some fiberglass cloths may not bear the same coatings that you get with the stuff CLC includes. Those coatings help 'wet out' the 'glass weave once it's saturated with epoxy. That could account for what you're seeing but without knowing what product you used and where it came from that's just a guess.

RE: Fiberglass Visibility Question

   Well that's quite a relief. Thanks a bunch. I remember reading about the "wet wipe" test to see if it'll go away. I didn't consider it after sanding. So should I do a light scuff sand now or wait for it to completely set up (a few days)? 


RE: Fiberglass Visibility Question

Depends... on what your ambient working temperatures are (affects how quickly an epoxy mix will cure hard enough to sand), what hardener you used (fast will cure more quickly at lower to average temps, slow takes more time but not if it's warm - like 75° & up - where you work), what your next step will be.

Scuff sanding's generally done before your first prime or varnish coat. All you're doing is breaking the surface gloss so the stuff you're going to apply will have a rougher surface (microscopically) to adhere to.

I've found that 'scuffing' a final epoxy coat with ScotchBrite material (whether from 3M or another brand like Norton's) is easier than using sandpaper, and can be done wet (with water from a hand-pumped sprayer) to keep dust under control. This also removes any contaminants that may have settled on the surface since it was coated last.

Sanding an epoxied coating in preparation for ading more epoxy ought to remove more material than simply scuffing; your primary goal is removing high spots while roughening the lows so an additional coat will better fill the lows while leaving less on the highs.

Whatever degree of sanding you accomplish, a quick wipe with a cloth & alcohol removes sanding dust left after vacuuming, gives you an opportunity to evaluate your surface for defects before deciding what's next. You can use clean water too if you'd rather but ensure adequate time for it to dry thoroughly before your next coat of whatever....

RE: Fiberglass Visibility Question

   So I did a fair amount of sanding and could see all of my low and high spots in my epoxy coat. I got it nice and even and wiped it with denatured alcohol. I then put on another thin layer of epoxy and it looks identical to the picture I attached. The epoxy finish itself looks much better after the sanding, or course, but the weave is still visible. The surface of the epoxy is almost mirror smooth and there is no pattern printing through. It is just that the weave appears white instead of clear and it changes sheen depending on the angle I view it. 

I tried to isolate variables so I tried pre-wetting a piece of scrap wood, then put a small piece of cloth on it and wetted it out to make sure the wood wasn't absorbing too much epoxy. No change; still visible after multiple coats, sanding, and an alcohol wipe. 

I put a few coats of epoxy on the fiberglass tape at the fillets and that looks pretty good. The fact that that is turning clear and it is much heavier weight and I put less effort into it looking perfect leads me to believe that the cloth itself is an issue. 

I'm thinking maybe I should just buy enough cloth from CLC to do the outside. I don't want to save $80 and have a weave visible everywhere. For what it is worth, I attached a link to the cloth that I am currently using. I know very little about cloth (obviously) so I don't know how to compare this to what CLC sells.

Thanksk again for your advice. I'm enjoying the learning experience even if I'm a little anxious about getting this right. 


RE: Fiberglass Visibility Question

Hi Brian, 

there are differences in fibreglass cloth that can explain what you are seeing.  if you want a glass that is confirmed to wet out clear, i would buy from CLC or a manufacturer/distributor (e.g. jamestown distributors) who can confirm the glass's properties for clarity after wet out when being used with dietnfiied brands of epoxy.

the link from amazon is showing a glass the is from  'fantasycart' and they look like a random reseller and there are no technical specifications on the quality or manufacturer of the glass.  so while not obviously the wrong material, there is simply not enough information to confirm that it is the right material absent your own experimentation. of the commenters on the feed back for the product notes that it does not wet out clear....which may be a hint.

again, not necessarily bad stuff and i am confident that you are structurally fine....but the property of a very clear wet out is technically nuanced and i would stick to known epoxy/glass manufacturer-specific product combinations to ensure you get the that clear effect you want.

RE: Fiberglass Visibility Question

Yep, you may have gotten cloth which has a coating called a "coupling agent" intentionally designed for compatibility with polyester resins, not epoxy resins.  Some cloth has a coating to bind better w/ polyester, and it is NOT fully compatible with epoxy, and some has an agent for epoxy.  The whiteness of weave when fully embedded in cured epoxy may be a hint in that direction.   Google "fiberglass cloth coupling agent" for some articles both academic and practical.

RE: Fiberglass Visibility Question

Thanks for the tip. I learned (too late) about all of these cloth variables. To avoid any further issues I just ordered plenty of cloth from CLC. It was just such a shame since everything else was looking so good. The epoxy was working well and wetting out great, other than the issues I mentioned. 

When I get the new cloth I'll test a little piece again to be sure that was indeed the variable and then I'll report back so that others can hopefully learn from my mistake. 

Thanks everyone!


RE: Fiberglass Visibility Question

Well, if it really bugs you seeing that texture inside, you could strip it all off & start over. Heat gun & patience'll undo epoxy.

But as it's on the inside I'd just chalk it up to experience, confident in thr knowledge I'd gained from the doing, and that it's not gonna be seen by anyone but myself or a close friend perhaps.

RE: Fiberglass Visibility Question

And that link you posted shows a "frequently bought together" Amazon bundle clearly featuring polyester resin, a hard-to-ignore clue that's what the cloth you used is meant to be used with... maybe.

Best buying materials from trustworthy vendors who can answer technical questions about the products you're considering if their websites may not be as information-rich as you'd like.

RE: Fiberglass Visibility Question

I like the idea that I bought the wrong cloth. I was fastidious in reading and watching videos ahead of time as to how to apply it and I think I did a pretty okay job, which is why it was so disappointing. I'm glad that I now see the polyester resin and that it is likely that. Now that I look at various reputable websites, if the photos of the cloth are to be believed to be accurate, it looks MUCH more transparent than mine. Mine has a very tight weave and it very opaque when dry. Hopefully that's a good sign. 

I definitely won't strip out the interior though the thought came to me. I'll just complain about it and shake my head every time I step in. It certainly doesn't look "bad". It's completely even and uniform, not splotchy or anything (which is why I didn't think it was my technique). 

Now I'm anxious to get the good stuff in so I can get on with the project. 

RE: Fiberglass Visibility Question

"Boat: a hole in the water, lined with wood, into which one pours endless sums of money."

...a paraphrase of a quote by somebody I don't remember, encountered somewhere I don't remember....


RE: Fiberglass Visibility Question

Success! I attached a photo of the new stuff from CLC. I applied the cloth and 3 layers of epoxy with exactly the same technique as my original picture so you can see that it is quite a difference. What a relief. 

I also attached a side-by-side photo of the old cloth and CLC cloth so you can see the difference. I'm mostly doing this for any future boat builders so that they don't have the same problems I do. Lesson learned. 

Thank you very much to everyone that posted. This was a great learning experience and saved me a ton of time and potential disaster. Thanks!

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