Cold Weather FG Repair

[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