spider cracks in gelcoat

-- Last Updated: Mar-09-07 10:07 PM EST --

Hi, I just purchased a 24 year old fiberglass canoe for cheap. I have
repaired two holes with west systems epoxy and plan to paint with
interlux brightsides linear polyurethane. My prob is as I started to
sand one of the holes I repaired with the orbital sander, there has
appeared several other star shaped cracks in the gelcoat. There is a
bit of flex in the canoe hull where these showed up. Do I have to
grind them out and epoxy or will the poly paint act as a sufficiant
coating for calm river and lake use? Thanks

If you have dried the boat out very
thoroughly, you may get away with just painting over the star cracks. I would suggest you consider a sealing paint on the inside also.

However, there is a reason for not painting what is (probably) a polyester resin boat of unknown quality. Older resins are subject to “hydrolysis,” a chemical reaction between the resin and minute amounts of infiltrated water. Hydrolysis weakens the resin, and can cause blisters in overlying gelcoat, or paint.

My experience with an old Mad River canoe, that developed extensive hydrolysis, suggests that the best way to avoid it is to bring the boat to a dry place after use, and give it plenty of time to dry out. Putting a tarp over it will tend to accelerate hydrolysis by holding hidden water in the layup. Allowing prompt and thorough drying will help minimize hydrolysis.

The West solution to hydrolysis is to hog out all the blisters and star-cracked areas, fill in with resin, and then coat the entire hull with epoxy resin. Expensive and not necessary, IF you dry the boat out. You don’t need to bake it, just put it where the humidity is low and the temperature is enough to promote deep drying.

Some polyester resins don’t tend to hydrolyze. Vinylester resins are stronger and have nearly no hydrolysis. Epoxy resins are a little stronger than vinylester and hydrolyze hardly at all.

I’ve gone through this because, given that you have a 20 year old boat, it may always have been stored so as to dry out in the past, but if you paint over the resin, you may trap water in there and end up with blisters and weakened resin.

Mad River resin
Mad River, before the move to the great american south, used Dow Derakane 8084 resin, a byphenol based epoxy - vinyl ester. Nasty stuff to work with.

Boat is dry
Thank you, The boat has been in the cellar for a month and should be completely dry. I just did some more sanding and a new bunch of spider cracks almost overlapping - about 4 inches in length and one wide just -started up - and yet another one about 6 inches from that. Maybe the gelcoat has been damaged from being stored in the sun over the years by the previous owner. He had it on a rack outside when I picked it up.

Do I have to worry about them or future cracks if I paint the outside and inside? The qt of west systems cost me too much already :slight_smile: so I don’t think I’d want to go through that expense for the canoe since I just wanted it for a temporary until I can find a used lightweight royalex solo model.

My old MR was laid up in '73, and
from the extensive hydrolysis, I doubt they were using vinylester at that time. Jim Henry specified some sort of isopthalmic polyester.

Sounds almost like the gelcoat is
drying out. Hard to explain the appearance of new cracks unless they were hidden previously by swelling of the gelcoat.

The exposure to UV from the sun
Has caused the resin in the gelcoat to de-polymerise. Unfortunately there is not a lot you can do to reverse this effect. I would ensure the boat is really dry then paint first with a primer, more than one coat if necessary followed by the topcoat.

Use marine paint, (interlux is a good brand) not general paint.

Let us know how you get on.

Couldn’t the cracks have appeared
from my aggressive sanding and the flexing of the hull in that area? The hull was flexing at least an inch and the cracks are just this side of the chines on the keel side

Yes they could have
Particularly if the gel coat is brittle due to UV rays & age.

Try less aggressive sanding, it may take longer but you seem to be making the problem worse sanding the way you are.

sand it
Sand the whole boat down. The cracks could be stress fractures from use (bumping rocks or even dropping it). the only way to tell is to get to the fiberglass and see if there are fractures in the glass itself. Then fix all the glass fractures first then gel coat (marine grade). That will ensure that you have a smooth crack free surface. I’m sure that if you are not experienced painter. that you could find some one to gel coat it for pretty cheap. there is not any need for fancy epoxies or paints. Marine grade gel coat is used on all fiberglass boats and is quite simple to spray and buff out.