Building Tips and technical articles. > Paint and finishing
ZAP FINISHING RESIN NOT HARDENING
Brett Buck:
--- Quote from: Motorman on April 24, 2021, 05:47:15 PM ---I use Zap Zpoxy finishing resin and haven't had that problem. Do you weigh the portions, did you accidentally mix it with your 30 minute epoxy? I think the instructions say too much hardener will turn it to rubber.
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Too much of either will have about the same effect, if it is epoxy - when mixed properly (most thin surfacing epoxy is 2:1 or 4:1) there are just enough reactants to react with the other part, and that will maximize the hardness and strength. Anything else, either direction, and you leave unreacted fractions of the one you had too much of, and it just softens it up.
Finishing resin (polyester) is typically a few drops or dabs of hardener for a large amount of other part, and you can control the rate at which it sets, more hardener, the faster - and hotter - it gets.
Brett
bob whitney:
been eye balling it for ever unless i am mixing a large batch (zap Epoxy) .mixed 2 batch's 15 cc each 50/50 mix. left it in the cups and neither hardened,2 days now i had a qt can of my prop resin that had a green color when i got it and should have been blue,it would not harden. they replaced the qt can for me and the new blue hardener worked fine
Brett Buck:
--- Quote from: bob whitney on April 24, 2021, 06:20:02 PM ---been eye balling it for ever unless i am mixing a large batch (zap Epoxy) .mixed 2 batch's 15 cc each 50/50 mix. left it in the cups and neither hardened,2 days now
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Sounds like the hardener went over the hill. I think the type or hardener used for surfacing is more-or-less the same stuff as regular 1:1 mix, just without the filler* to get the mix even. But it seems to me much more aggressive/reactive, and I have had it destroy the bottle it came in, apparently, working on the unreacted components in the polypropylene bottle, making it brittle to the point it shattered when I picked it up.
Sorry to hear this, it is a *real mess* to try to get off. I would try moderate heat for a long time, essentially, accelerated aging.
Brett
*p.s. one of our composites guys at work told me that for consumer epoxy, they put inert fillers into the hardener to buffer it, and to make it come out 1:1 for easy mixing - and that one of the common fillers is *castor oil*. That's why most of it is a little flexible/gummy and hard to sand. Most of the "customer-mixed" industrial stuff they use is more like 16:1 or something like that. But most of what we use in our aerospace application is premixed at the factory - mixed carefully under controlled conditions, then frozen to keep it from curing. To use it you thaw it out, apply, then it just cures like a one-part glue.
Jerry Rauch:
I worked for a race car chassis builder as a fabricator for 8 years. One of my jobs was to make flat carbon fiber sheets for building race car interiors. This was done using vinyl ester resin and MEK as the hardener. These sheets were about .030 thick and were 5'x10'. Yes, that's feet! Used a sheet of 1/4" 5'x10' glass for the "mold". Because of the size, didn't have much time to lay up. Anyway, the resin mix was 3/8 oz. of MEK to 1 quart of resin, which would give about 25 or 30 min. working time and each sheet took about 4 quarts total. One time the weather here was cooler than usual, and this sheet was not getting tacky on the back like it usually did after 30 minutes. Because the table the glass was on, had wheels to move it around, we pushed the table out into the sun to speed up the cure rate. Bad, really bad, it cured alright, and cracked the $400.00 sheet of glass, and the carbon sheet was stuck to the broken glass for good. So, we were out the glass, and the $1,000.00 sheet of carbon. Come to find out the supplier shipped us MEK that was a higher percentage than the usual stuff had. We never did find the reason the sheet wasn't curing at first, though, but did find that little extra heat from the sun attacked the 4 coats of mold release wax on the glass. I don't remember what the percentage difference was, but it wasn't much. Any fiberglass resin, whether vinyl ester or epoxy based, is extremely sensitive to mix ratios. I don't see how, with small mixes like 4 or 6 ounces of resin could a person accurately measure the MEK needed.
I do remember that the MEK, when we would get close to the bottom of the gallon, the cure time was longer, but we always used the same ratios. I guess the MEK got weaker from age.
I would be real careful in glassing a plane, I would use only new, unopened resin and hardener. I saw a lot of weird things happen in fiberglass work.
pmackenzie:
MEK is a solvent.
Pretty sure you were using MEKP, which is the catalyst used in polyester/vinyl-ester resins.
Very different reactions going on in catalyzed curing(polyester) versus stoichiometric (epoxy)
MEKP is pretty dangerous if you get it in your eye, much more than most other things we mess around with.
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