I have done test projects both ways (with a sealer coat of epoxy and without). The only noticable differences were...

1. The un-sealed project had a lot more bubbles comming up under the glass. I was out there popping them every 15 minutes for hours, and I did miss some.

2. The un-sealed project used more extra epoxy on the glassing step than the sealed project used on the sealer coat.

P.

> The technique will probably save a few pounds, however it is
> risky. I no longer seal the wood. The reason epoxy manufacturers recommend
> a seal coat is to get good penetration into the wood. Some manufacturers
> have even recommended thinning the epoxy to get more penetration.
> Part of the strength of the wood glass composite comes from the resin
> soaking into the wood, so you do not want to limit too much. Limiting
> it slightly is probably not a bad thing.

> A potential problem is you must let the seal coat cure before
> applying the glass. You will probably only get a mechanical bond between
> the seal coat and the glass coat of resin instead of the better chemical
> bond. This can be avoided by applying the cloth coat while the seal
> coat is still green, but you will probably want to sand off dust-picks
> which will snag the cloth. It is harder to sand green epoxy.

> If you wait for the epoxy to cure fully, you risk blush. You
> must remove the blush completely. My brother peeled the fiberglass
> off a boat built in a humid basement by grabbing a slight delamination
> at one end and pulling. In half and hour the glass was completely
> removed. Not a good thing.

> Contaminations falling on the seal coat can also create a poor
> bond. Oils from your hand which would be absorbed in bare wood stay
> on the surface of sealed wood and can make a weaker layup.

> If the seal coat is too smooth you will not get a good mechanical
> bond with the cloth coat. You need to sand the smooth surface to achieve
> the best bond. If you sand through the seal coat you can create blotchiness.
>

> I am currently fixing a boat where some of the fiberglass has
> delaminated from the seal coat. It took six years for the problem
> to occur, but fixing it in no fun.

> In my book I am a little wishy-washy about the seal coat, but
> now I do not recommend it. You may be able to save weight, but unless
> you are very careful you could end up with a substantially weaker
> boat. The modern low-viscosity resins soak easily through the cloth,
> so much of the reason for a seal coat has been eliminated.