Newbie to sanding fiberglass - questions/issues

Go easy on me please - I'm new to boatbuilding :)

I'm building a Sassafras 12, and just finished glassing the inside bottom of the boat.  

It was my first experience fiberglassing and I didn't squeege out as much epoxy as I should have - so I have some areas that floated a bit.  I've got two coats of epoxy on top now and just started the sanding.

Here's what I am seeing for the most part after about 20 minutes of sanding - I'm assuming THIS part is as expected - lots of high and low parts, and lots of sanding necessary to smooth it all out:

But now the questions start :)

First, what's the RIGHT way to be sanding on the inside of the boat?  I have a nice 5" random orbital sander, but it's too big as the strakes get smaller near the ends.  Should I just use a rubber block with sandpaper and do it by hand, or a foam pad on the random orbital sander?  

Second, how obvious is it when I get down into the weave of the fiberglass?  I think it is VERY likely that I will given I had the glass floating in some spots, but I'm wondering how obvious it will be?  I can see spots like the one below, and I can see the pattern of the weave - am I down in the glass here already?

This one isn't as clear, but the splotchy oval in the bottom is smooth - which is maybe the edge of the fiberglass?

So I guess that's my fear - will it be pretty clear when I'm down in the fiberglass weave when sanding?  And, given obviously my fiberglass job isn't great - what should I do - keep sanding until it's smooth even if I'm in the fiberglass?  Or build up a few more layers of epoxy and then sand, or fiberglass another layer on top of this?  


Thanks in advance for the advice - it will be greatly appreciated!

11 replies:

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RE: Newbie to sanding fiberglass - questions/issues

In the last picture that spot in the bottom left does look like you got down to the fiberglass and removed some.  Often you will be able to feel and see fibers around the edge of a spot like that when they first develop but it looks like you sanded past that stage in that photo.  A little spot like that on the inside of the boat I would not worry much about, just try not to make a habit of it. 

Hard to tell without seeing it but you might need to just scuff what you have fiberglassed and re-coat with more epoxy to build the low spots up.  Your boat will end up slightly heavier but no big deal.

RE: Newbie to sanding fiberglass - questions/issues

Edit: meant to say spot in bottom right.

RE: Newbie to sanding fiberglass - questions/issues

I agree with atwoodyducker. Even if you cut completely through the glass on the inside of the boat, it won't affect the strength of the boat.

One essential piece of gear for boatbuilding is a thick foam pad for your sander. It fits between the disk and the paper, and allows you to better fit curved areas.


RE: Newbie to sanding fiberglass - questions/issues

Thank you very much guys - it sounds like I'm not in too much trouble then :)

I think I will continue to sand (avoiding spots like this where I'm down into the glass) - and it should be pretty clear after some more sanding where I have low spots that would benefit from more epoxy.  And I will give the whole thing another coat once I have things smoother.

Thanks again - I'm off to the hardware store at lunch to look for one of those foam pads for the sander!


RE: Newbie to sanding fiberglass - questions/issues

Be careful with epoxy dust. This from "Never breathe the sanding dust of partially cured epoxy. Epoxy chemicals remain reactive until they have cured. Serious health problems can result from sanding epoxy before it is fully cured. When you inhale these dust particles, they become trapped in the mucus lining of your respiratory system. The reactive material can cause severe respiratory irritation and/or respiratory allergies."

RE: Newbie to sanding fiberglass - questions/issues

Let me second that. Buy a $50 3M respirator--forget those paper hankie dust filters on a rubber band.  Sanding work is what its all about and your nose is often inches away from the surface. Epoxy dust is bad, but even sawdust can put you to coughing up junk for a week.

RE: Newbie to sanding fiberglass - questions/issues

I'm having a similar issue and wanted to make sure I understand the answers (I realize this is very old, so new replies from other people are appreciated. Thanks!).

I have fiberglassed and filled the weave, let it cure, then sanded it down. The panel is no longer shiny (except for little tiny patches). Because those patches are still there I find it hard to believe I've cut into the fiberglass... yet I can see the weave just like in the second picture above. The wood is smooth there, I can't feel any fiberglass fibers. There ARE places where I've clearly sanded into the glass, mostly on the edges: it is very white colored, cloudy, and a little rough feeling.

So my question: is it normal to see the weave pattern through the epoxy, or have I over-sanded (or under-epoxied to fill the weave pattern?). Note: I'm painting, so aesthetics are not important - just strength is.

RE: Newbie to sanding fiberglass - questions/issues

The easiest way to tell what's going on is to wipe the area with a wet rag (either water or denatured alcohol). Anywhere the epoxy turns transparent and the weave disappears is fine - you're not into the glass yet no matter what it looks like when it's dry.   If it's white and/or you can see the weave while it's wet, you're in the glass.



RE: Newbie to sanding fiberglass - questions/issues

  Laszlo - Perfect, that's exactly the info I needed. Thank you!


RE: Newbie to sanding fiberglass - questions/issues


a very useful tool in the 'sanding' arsenal is an inexpensive cabinet scraper.

after filling the weave, but before starting with a ros sander, a cabinet scraper can do a great job of knocking off the high spots.  it creates a lot less dust and is easy to control.  it can save quite a few sheets of sandpaper that would be used to get to an equivelant state with sandpaper..

I am currently at about the same stage you are on a strip-built kayak and was scraping last night with great efficiency.  i recently picked up a new set of scrapers from Garlick Saw Company  that is designed for working on the inside of curved surfaces -- their Shaped Cabinet Scraper Set -- which is awesome for handling  getting into those inside-of-the hull curvy sections.

best of luck.  your pictures suggest to me you are doing just fine.


RE: Newbie to sanding fiberglass - questions/issues

  Laszlo this test from years ago just saved me a lot of anxiety and confusion. Thanks!

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