Hi Andrew, 

if you have done three coats, you are probably fine.   you are not seeing the fibreglass if you have three coats already on there - you are seeing the print through.  the question is, is there enough epoxy on top of the glass such that when you sand it, you can get it smooth without cutting into the glass?

how you tell is a bit tricky if you have not worked with epoxy and glass before.  

this you tube video:   https://www.youtube.com/watch?v=-qTctYgHwqo     should give you a sense of the look of what it should look like as 'filled' prior to sanding.

after that, you just have to start sanding.  if you sand and cut through to the wood, you have gone too far.  i would describe what you are looking for is you sand you can get it smooth and you don't see a sort of whitish cloth pattern that appears when you cut into glass vs just into the epoxy on-top top of the glass.   however, as the treatise says, it is not a problem if you cut into the glass a little bit...as long as you don't go all the way through.  this other video i think you will find useful on sanding:  https://www.guillemot-kayaks.com/strip-built/recreational-kayak/petrel-play/making-petrel-play-outer-seam-final-sanding-and-prep-e17

hope that helps

h

   Thank you!

Another newbie question along the same lines: as I sand epoxy, how can I tell if I sanded through to the bare wood?

Hi Andrew, 

that's a good question.  

the wood, when it has epoxy on it darkens slightly and will have a shine.  so if you go through the epoxy, you will see a distinctive 'lightening' of the color where you sanded through.  it will also lose its shine (because now it is just sanded wood)

if you proceed much beyond that, you could sand through the veneer into the core of the plywood....and that will look dark/blackish.

one of the things i like to point out is most kits have a little bit of scrap wood.  you can always do a little experiment on one of these pieces (glass it, sand it, sand through it, etc.) to play with some of the ideas in a way that doesn't put your bigger project at risk.

h

   That's a great idea, I should've done it long time ago! (I don't have scrap wood in my kit, but I do have a motor pad which I don't need.)

Now, when I have the whole boat epoxied, and the weather is getting warmer, I am getting really impatient to finish the boat.

I am curious how important is to sand it really neatly. Is it mostly a cosmetic thing, or the varnish would really adhere to any patch of not-thoroughly-sanded apoxy?

Hi Andrew, 

the short answer is, it's mostly a cosmetic thing.

like you, i like to use my boats and be out there so i do not obsess about it.   i sand it reasonably smooth, varnish trying not to be unreasonably messy, and go out and paddle.

from where you or others will typically look at it, you will not see any of the imperfections you might be able to detect at two feet away or closer - which is about the distance i consider when somebody is really inspecting things carefully.

this boat pictured below for example, which i paddle several times a week, has various runs, drips, patches and other imperfections in it's epoxy and varnish work that you can detect from within two feet if you were looking carefully.  but this shot is from a little over 10 feet away with a high definition (lots of pixels) camera.  can't see any of the imperfections....and this is what any guest or other person on the river sees.

so unless you are trying to win a close-in competition for best sanding/varnishing job in the world, i would get out and have fun paddling.  this approach also results in lot less mental anguish when you get the inevitable ding.  who needs mental anguish?  you are trying to have a good time!

   Yeah, totally agree. I was just concerned about varnish not holding to the epoxy. If that's not a concern, it already feels liberating!

Your boat looks beautiful, by the way. Can't see any epoxy runs at all :)