Fiberglass blemishes

Hey all, thank you for the help in advance...

I recently completed my first cedar/redwood strip canoe. I just started noticing some weird subsurface blemishes in the fiberglass. The marks concentrate around the redwood strips on one side around the middle of the hull around the water line. Also, this is only on the outside, there are no indications of damage or marks on the inside.

My best guesses right now are the following but none of these make much sense to me...I'm sure I'll be sanding and patching the fiberglass soon but want to see if anyone has any ideas on root cause.

1. poor epoxy adhesion

2. Heat - its been pretty hot and the canoe has sat on the top of the car a couple of times for a couple of hours in ~90 deg heat and full sun

3. When strapping the canoe down I'm somehow putting some weird stress on it which is cause the glass to pull away from the wood.


I used MAS epoxy and 6 oz fiberglass cloth and Total Boat Gleam 2.0 Varnish.

4 replies:

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RE: Fiberglass blemishes


i looked at your pictures and i have had something like that happen on one of my boats. 

i think you have one or both of the first two issues you mentioned, poor adhesion and heat.

on heat, in the direct sun, the surface of a boat can get very hot.....and you can see this happen.   i am pretty careful about leaving my boats in the sun when it gets north of 90....which is pretty common where i live.

this may also be exacerbated if the epoxy did not get a good cure.   with the boat cool, probe the surface by pushing a dull nail (or fingernail) into the surface.  if you can mark it.....your epoxy did not fully cure.   

let me know what you find on the second test...if there is an epoxy cure problem, i can make recommendations as well.


RE: Fiberglass blemishes

   Thanks for the reply. I just pressed on some of the areas with a small screwdriver and it is rock hard so curing doesn't seem to be the issue. 

I'm going to keep an eye on it and see if it spreads and keep track of environmental conditions and see if I can track down what it causing it. 

One odd thing to me is that it seems to be concentrated around the redwood strips. The second picture I posted originally has 3 thinner strips or redwood above the dark cedar.

One more thing...the surface is raised as if the glass is being pushed up. The wood that I used was very dry, I don't have an exact number but was thinking if the heat caused some of the moisture in the wood to cause a bubble. I don't know if this is possible but something I thought of...

Ultimately, I'm trying to not get too worried about it since the wood is still dry (no fiberglass breach) and it isn't happening on the inside (still structurally sound). I'll definitely be sanding and reglassing that area in the future. Do you see anything in the pictures that would cause immediate alarm to you?


Thanks again


RE: Fiberglass blemishes

nothing that would cause immediate alarm.

when the wood gets heated it will outgas (expell gas) even if the moisture content is zero.  the wood has a lot of air in it even if it is totally dry.....that's what creates the pressure/small bubbles.

the concentration of bubbles along the redwood, to your point, is probably for a reason and not random.  but why is a bit hard to say.

when you strip the fibreglass off a boat like this using a heat will see something similar to what your picture showed as the first indications that you are getting the boat hot enough to pull the glass up....



RE: Fiberglass blemishes

   Interesting, thanks for the info. I had no idea you could strip fiberglass with heat. Hopefully I won't have to do that anytime soon.

Any ideas on how to keep a boat cool on a roof rack in the hot north carolina sun? Might just have to be one of those things I stay cognizent of. Thanks for the info!



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