Fixing Voids Below Gelcoat

Barnacle Bill · May 7, 2012

It would be helpful if we had a bit more information such as how big are these areas and is there a core material in the matrix? Epoxy is expensive and not compatible with polyester gelcoat, so you might want to do the repair with polyester resin. But, need to know more info. For example, if you have wet core beneath, then that has to dry out first and the only way to do that is tear into the surface, etc. Offer more info please.

Indysailor · May 7, 2012

I experimented with using polyurethane glue to deal with the area around my starboard chain plate, which was soft to a radius of about four inches. By compressing the area, I was able to control the foaming of the glue so that it spread into areas where the core was soft.
It worked really well on my little job and I'll evaluate it regularly to see if it may be a viable solution for other areas. My main concern is that the glue may not really bond to both the upper and lower skins, but rather is only creating a temporary hard spot. Time may tell.

I'm not saying this is a preferred method over taking off the upper skin and replacing the core, especially if the problem is wide spread. But it may work in the right application.
Here is some information on injecting polyurethane.
http://www.sailtraininginc.com/repairs/balsacore.htm
http://www.sailtraininginc.com/repairs/balsacorerotsummary.htm

kenn · May 7, 2012

Drilling access holes into the void and squirting in some epoxy might be reasonable, if there are no other problems. Anything wet will have to be cleaned up and dried out, and if the void has no backing, you may have to patch first from behind, or use thickened epoxy that won't run out. I patched a few parts of our cockpit sole in this fashion. Epoxy needs to be protected from UV, so be sure to top up the access holes with gelcoat.

Barnacle Bill said:
Epoxy is expensive and not compatible with polyester gelcoat, so you might want to do the repair with polyester resin.

Once polyester is fully cured (beyond a few days), this isn't the case, and epoxy will actually bond slightly better than polyester, since it's now just a mechanical bond. I believe the West System epoxy guys did a full series of epoxy-to-polyester bond testing to demonstrate this. But polyester resin will also do a good job here.

I've gone through 3/4 of a gallon of epoxy in 3 years, doing some structural repairs. You won't need quite that much

higgs · May 7, 2012

I have had two boats in the past where I injected epoxy into some voids beneath the gel coat. both times the repair worked great and both boats gave me many years of service with no additional problems.

RichH · May 8, 2012

If you inject epoxy ... the normal 'fix' for voids, you should use a very slow cure epoxy to prevent the mix from becoming 'exothermic' which will push the void apart. Even with the use of very slow cure epoxy, its a damn good idea to have ice on hand so that even if the slow cure epoxy 'kicks too fast' you can 'slow it down' by the application of ice to the surface. When you mix such epoxy, keep it THIN (thickness) when mixing, and cold (on an ice bath under the flat pan, ... not a 'tub', you mix it in) ... to help it from 'kicking' too fast. Keep it COLD before, during, and after the mixing and injection.

Two holes are usually needed: one hole for the 'injection' and the other 'smaller' hole as a 'vent' .. and simply fill until epoxy exits the 'vent'. Such voids are common, and usually are not filled with 'water', etc.

A goodly supply of white vinegar on hand, in case the injection 'kicks' too fast and foams out all over the place ... will help STOP the kick and allow easy 'clean up' of the 'foamed out' MESS ... IF the mix goes 'exothermic'.

Bob J. · May 8, 2012

Rich,

To slow down the cure, can you use bit less hardner?

Bob

RichH · May 8, 2012

Nope, not with epoxy. Epoxy REQUIRES a fixed ratio of 'hardener' to resin.

To do this with epoxy you really have to use a SLOW cure mixture using the proper slow cure 'hardener'.

With 'polyester' you can vary the amount of MIBK ... but you really have to do a LOT of small batch sampling first, to get the slow cure time and the proper amount of MIBK correct.

Robert M. · May 8, 2012

Ducati said:
Tapping on the gelcoat reveals air pockets and when crew step on it cracks occur.

I was thinking of mixing up some West System resin + hardener drilling a few holes and injecting the stuff to fill the void.

Once dry I can fill the holes with gelcoat and wet sand.

Does this sound like the right method for this repair?

Thanks

Briefly, here are the steps:

1. Drill two small holes into the void. They should be located at its lowest and highest point.

2. Introduce pressurized air into the void to purge it of any moisture.

3. Mix a batch of polyester resin (isophthalic) using a very small amount of catalyst (MEKP), 0.25 - 0.50 per cent by weight.

4. Slowly introduce the resin into the lower hole until it flows out of the upper one.

5. Temporarily plug both holes and wait for the resin to completely polymerize.

6. Permanently seal both holes with polyester gelcoat (NPG ISO).