fiberglass help


New Member
Hello people. I am new here and in need of some help. I wet out the fiberglass on my 18.5' canoe. I sanded it and went to put a filler coat on today and i had some white spots show through, I wiped it down and cleaned it and they still shown through, There were not many at all. Just a few spots. I went ahead and put a filler coat on it as i did not think they looked that bad though they are disappointing. Can you guys take a look and tell me what you think the problem was/is. Not sure how to post a picture here so if i may i will post a link to were i have the pictures at. . This is the first canoe for me and there will be many more to come.
( update ) the little round ones did not show through the filler coat. The square ones did ;-( any ideas? ot am i being way to picky, It looks good and only noticeable when you within a few feet of it a canoe/

thank you
I'm no expert, but it looks to me like you squeegee'd too hard,and starved the weave -- took too much resin out of it, leaving air pockets. Hopefully, someone who knows better can clarify this, and give you some tips on how to clarify that...

Does it look better, now that the filler coat is set up?
I can't see the pics but usually white spots/areas are places that don't have resin between the cloth and wood.

I've filled them in the past by drilling a small hole at each end of the starved area, and injected resin in 1 hole till it came out the 2ed. Usually the white "goes away" when the resin fills the cavity.

You might have to do this at several locations to fill a larger area.

I use old syringes for this, they work great.

I can see them. Those little white spots are not from over-squeeging and/or being resin starved, and they're not tiny bubbles. They are simply spots where you sanded down into the weave. There should be no sanding going on at that point for several reasons: (1) you want to leave the fiberglass on the boat, not sand it away. (2) the introduction of dust (even dust made from the same stuff) plus a bit of residual sand, sandpaper adhesive and anything that happens to fall or condense out of the air at that point is a very bad idea and should be avoided whenever possible. (3) The resin is not usually cured hard enough at this point for sanding and the heat and pressure from it can micro-fracture the fibers and the resin that is saturating them. This usually isn't a major problem, strength-wise, but it is what causes the variety of little white spots that never go away. There are tiny little fractures down deep in the fibers in those spots and nothing you could ever put on them will get that far in, re-saturate them and hide them.

If you really have to sand something before adding the filler coats (like feathering out some sort of reinforcement layer, for example) it would be very wise to wait a week or so for the fiberglass to really cure hard before you start sanding. When you sand into the glass after proper curing, it will still show the little spots, but they're just temporarily abraded on the surface, not fractured down deep. This type of white spot will go away with subsequent coats of resin, or even the final varnish. After that, they are nearly invisible, though they can sometimes temporarily show up as a patch of tiny silvery sparkles when bright sunlight hits the area from just the right angle. Not a big deal. Many of us never sand or feather anything (including things like multi-layered stem reinforcements) until all the pieces, and then all the filler coats, have been applied and cured. Any sort of stair-step effect between layers is, first of all, reduced drastically by the filling process, and what's left generally sands smooth quickly without risking tiny fractures. (4) sanding fresh epoxy is bad for you and one of the better ways to develop an allergy to it. If that happens, you're likely done using epoxy for good.
Good catch Todd,

Does this mean that this statement is a bit of an understatement. :)

"I sanded it"

Someday I'm going to have to get web access at home so I can see pics folks link to. :(

Yeah, that was the original "eyebrow-raiser" and the pics were pretty obvious. Fiberglass cloth really isn't flat. The up and down of the weave tends to make a surface that is actually a series of very tiny, evenly-spaced peaks and valleys. Cut the peaks off a little bit with a sander and you have a patch of small white spots in a grid pattern. Cut deeper and the dots get bigger and more square in shape. Do it when the resin is too green to be doing much, if any, sanding and some of them will always show, even after recoating.

We learned this the hard way back in the dark ages when we switched from polyester resin to epoxy. You could glass a stripper with polyester and the next day it was really hard and ready for any heavy sanding without a problem. When we tried the same thing on epoxy, the heat and pressure moved the fibers enough to cause the patches of micro-fractures. The big disk sanders that we normally used for sanding would actually smear the resin. We had to go out and buy slower disk sanders to reduce the heat buildup when sanding and start waiting the better part of a week for the cure to finish before sanding. With smaller, slower sanders you might not need to wait quite that long, but it can still be a problem at times (even a scraper can sometimes do it). You may not get much done during the days while you patiently wait for the final cure, but it's usually time well spent in terms of avoiding problems and final finish quality.

Interestingly, I had one double kayak that developed the mini-fracture patches and some obvious weave pattern all by itself, a few years after it was done. I got a new garage and decided to hang it up in the rafters. It seems that it got so hot up there during the summer that the boat's core wood expanded and stretched the outer glass layers - not enough for serious structural damage, but enough to create some areas with patches of the little white fractures and a bit of weave texture (once you stretch fiberglass, it doesn't go back). At that point the rafters became off-limits for canoe storage.