Cold Weather FG Repair

[ksreynolds74]

Two quick questions on a project boat I'm working on:

1. I'll try to get a picture of this one...I'm repairing two failed repair jobs in the bottom of my hull. I have floor & stringers out, so access isn't a problem. The two repair areas are about 24 inches long and 5 inches wide. They are ground out and tapered, and are bisected by the keel. There is about 3-4 inches width of keel. I can NOT get to pink glass. The glass in the keel is all dark, which I assume means its bad. Will the new glass bond to this and cause an issue? If so,I'm laying the holes up with melamine on the back side, so I was thinking of just putting the release wax on the keel (to keep the shape & line), then cutting it out from the bottom, and filling and fairing out from there.

2. What is the coldest temp. you'd recommend doing repairs with polyester resin in?

[VirginIslander]

If the repairs to this area have failed twice already, why would you be planning to use polyester resin? .... I'd be wanting to maximize my chances of a reliable repair by using West System or a similar epoxy.


Darlene

[bigeasy1]

I'll have to respectfully disagree with darlene,regarding polyester resin.Most likely,the reason it falied,was not because of the resin,but because of a poor repair job.No resin on earth,no matter how costly,will adequately adhere to a wet or poorly prepared surface.
Try finding any commercially produced boat line,even high end ones made with anything other than polyester,or maybe some vinylester resin.Even some with Kevlar,are still using polyester,vinylester,or some sort of orthothalic or isothalic resins You may find some built with epoxies,but they are usually reserved for exotics,one of a kinds,or race boats.
A repair done properly with polyester or vinylester resin,will be as strong as the surrounding area.as for temp,You probably wouldn't want to glass in temps any lower than fifty degrees or so.To do so could cause the glass to not cure properly,resulting in another failed repair.

[VirginIslander]

I absolutely agree that surface preparation and condition are key to any good repair.

Just as important in difficult repair situations is a well conceived plan of attack incorporating good mechanical understanding and the knowledge of stress points to be considered.

My point was, that all else being equal, testing that I'm familliar with puts polyester resin repairs at about 70% of the original strength, and epoxy repairs at about 85%. ... That's a better than 20% stronger repair for epoxy over ployester.

Down here in the Caribbean where I am, nobody uses polyester any more. The thinking is that outside of a production environment, it's just too unreliable by comparison.


Darlene

[NoBones]

11 cc's of MEK to 1 qt resin above 75 degrees. Under 70 degrees bump it to 13 cc's per qt. of resin. This should give you about 10 mins. or less to work the resin.
The hotter the mix the faster the cure. Try to wait for a warm day or use heat lamps on your work. Humidity also
plays a big part in cure time as well. The slower the cure the better the bond.
See ya, Ken

[bigeasy1]

Darlene,you make a good point about reliability with polyesters.Unfortunately,there's some poor quality resins out there,and people should be wary of resins that are called all purpose,or boatyard resins.
Usually,they are low price poor quality resins and they shouldn't be confused with high quality resins that are far superior.
I guess i tend to forget that,as i work for someone that only sells high quality resins,and i forget that there are some people selling inferior product.Thanks for bringing that up.

[oldbluesplayer]

Folks, I have to agree, and point out some differences, with all of what's been said - and, I come at this from the experience of being an engineer involved with the first fiberglass bladed aircraft propellor systems many years ago ( a forged aluminum hub-spar, shot peened, adhesive coated, with a molded on foam core, near net shape, with a sown glass cloth sleeve added, inserted into a mold, with resin vacuum drawn thru, and thermally force cured into net shape, with a metal leading edge bonded on after - beat the hell out of the hours of hand grinding the solid aluminum blades used since the WW II days - but it was so strange to see a bunch of lady seamstresses in the cutting and sewing room, off to the side of a hard core machine shop !!)

There are two major considerations - primary cure, and secondary cure: when a hull is initially laid up, it is all done in one big step, in a short period of time - all of these resins, in a primary cure, cross link to each other, creating a chemical bond to each other, as well as an adhesive bond to any dissimilar materials being used, such as a wooden stringer. This makes all of the glass become one solid, homogenous structure.

In a repair, the old glass is already fully cured, and you will get very little, if any, cross-linking between the new glass / resin, and the old - all you're getting in a repair is the strength of the patch, and the strength of adhesion of the patch to the original surface.

Therefor, in a repair, surface prep, and adhesion area, are of the utmost importance - thorough cleaning, plus sanding to give it a rough surface - this rough surface, giving it tooth, increases effective surface area, thereby increasing bonding area, and consequently, bonding strength. This is why, in fiber glass repairs, they say to bevel back the original surface to such a degree - to increase bonding surface area.

In a repair situation, epoxy resins have a measureably greater adhesion value, per given surface area, over polyester, as was pointed out by others, above.

In sum, what this means is - in the original layup, given a structurally sound design, a good fabric lay-up schedule, and proper reinforcement / stringer system, etc, it really doesn't matter what resin is used, it will all cure together and be, relatively, equally strong - yes, there is some minimal strength value differences between polyester, vinylester, and epoxy resins - but much greater cost differences.

But, after full cure - which for polyester resin is functionally about 24 hrs, and for epoxy is about 3-4 days (both are ambient temperature dependant) after full cure there will be no crosslinking, only adhesion, so in a repair the epoxy will be stronger than the polyester, given equal bonding surface area and surface prep.

On those times just mentioned - all of us that have done glass work know that the resins start to gel in about 10 minutes or so, depending on material, and appear to cure, ie, go hard, in about an hour - but that is not full cure, it is still what is called "green", and say for example with polyester, you can lay in some glass, come back in an hour or two, and sand and clean, and lay in more glass, and in this close time frame the second layer will still chemically crosslink to the first - giving both chemical and mechanical adhesion, the two layers become, chemically, one solid mass.

Temperature wise, I would want to do all resin / layup at temperatures of 55 F or above, although I note West Systems epoxys have special hardners available for use at temperatures below that, but I have no experience with their use.

just my thoughts

Bill