Epoxy still sticky after 2 days?

[billdz]

Hello,

Two days ago, I made fin fillets with 30-minute epoxy on one of my rockets. Now, 5 of the fillets and solid and smooth, but one is still sticky. Does this mean I got the mix wrong, not enough hardener? How to fix? Can I add a bit of hardener to the top of the existing fillet or do I need to somehow remove the sticky fillet and start over? Or do something else?

Thanks,
Bill

 

[tim cubbedge]

Was it a 1:1 mix? Might try hitting it with a blow dryer as heat usually accelerates cure time.

 

[billdz]

Was it a 1:1 mix? Might try hitting it with a blow dryer as heat usually accelerates cure time.

Thanks Tim, I'll try a blow dryer. Yes, 1:1 mix.

 

[Bill S]

Did you measure it by weight, say in a plastic cup, or was it eyeballing (I find this to be not very accurate)?

 

[billdz]

Did you measure it by weight, say in a plastic cup, or was it eyeballing (I find this to be not very accurate)?

I poured resin into a plastic cup, hardener into a similar cup, then poured both cups into a third cup. I did not weigh. I'm not sure how accurate this system is, since a substantial amount of hardener/resin sticks to the cup when poured, even after scraping it out as best as I can with a plastic knife. What's the tolerance, e.g., is the result bad if the mix is 55/45 instead of 50/50?

 

[afadeev]

Two days ago, I made fin fillets with 30-minute epoxy on one of my rockets. Now, 5 of the fillets and solid and smooth, but one is still sticky. Does this mean I got the mix wrong, not enough hardener? How to fix?

Yes, you did not mix equal amounts of hardener + resin.
If it hasn't hardened after 24 hours, it never will.
To fix, scrape out the gooey mess, re-mix by weight, re-apply.

Can I add a bit of hardener to the top of the existing fillet or do I need to somehow remove the sticky fillet and start over?

The latter.

I poured resin into a plastic cup, hardener into a similar cup, then poured both cups into a third cup. I did not weigh. I'm not sure how accurate this system is, since a substantial amount of hardener/resin sticks to the cup when poured, even after scraping it out as best as I can with a plastic knife.

Your method all but guarantees uneven mix, and results as observed.
Consider investing into a small scale, then pouring and mixing in 1 cup. Note the weight of resin you poured in, then weight an equal amount of hardener, then mix thoroughly.
The above approach works for me 100% of the time.

HTH,
a

 

[PSLimo]

Hi Bill,

Moving forward it's hard to beat picking up a $10 digital gram scale from Amazon zeroing your cup out and getting it perfect every time.

Had a similar incident 10 years ago and have been using the same cheap scale ever since without incident.

 

[billdz]

Yes, you did not mix equal amounts of harder + resin.
If it hasn't hardened after 24 hours, it never will.
To fix, scrape out the gooey mess, re-mix by weight, re-apply.

Hi Bill,

Moving forward it's hard to beat picking up a $10 digital gram scale from Amazon zeroing your cup out and getting it perfect every time.

Had a similar incident 10 years ago and have been using the same cheap scale ever since without incident.

Yes, thanks, I just did a mix by weight in a single cup, wish I had thought of it. This appears to be a much better method. I had been using the 3-cup method for 4 years, must have gotten lucky, because this is the first time I have seen the stickiness issue.

 

[aviserated]

Some epoxies have a shelf life. Epoxy will not harden if too old. I have never had this problem using plain old JB Weld.

BTW the over rated Rocket Poxy has a short shelf life I hear.

 

[neil_w]

I must be living charmed... I usually mix by eye and have never had a batch fail to cure properly. Note my quantities are usually pretty small.

I vow to terminate this bad habit.

 

[SharkWhisperer]

I must be living charmed... I usually mix by eye and have never had a batch fail to cure properly. Note my quantities are usually pretty small.

I vow to terminate this bad habit.

Likewise, though I have no plans to change this "bad" habit. Never ever had an issue, but I also use small quantities. Can't see how 45:55 would make much of a difference vs 50:50. Weighing epoxy is a pia.

 

[Bill S]

I poured resin into a plastic cup, hardener into a similar cup, then poured both cups into a third cup. I did not weigh. I'm not sure how accurate this system is, since a substantial amount of hardener/resin sticks to the cup when poured, even after scraping it out as best as I can with a plastic knife. What's the tolerance, e.g., is the result bad if the mix is 55/45 instead of 50/50?

I have no idea. I use one cup, pour X amount in the cup, noting the weight, add the other part, stop when its double the first weight. But I am not making and weighing large batches of epoxy either; they tend to be small batches for noseweight, etc.

 

[heada]

Read the instructions. Some are measured by weight, some by volume. Some are different than 1:1.

For example, West is not 1:1 by weight or by volume and the mix ratios vary depending on if you use additives or different hardeners.

Some epoxies require a post cure heat to fully cure.

In the end, if after 24 hours it's still sticky, I'd remove it, clean the area and mix a new batch.

 

[prfesser]

Some epoxies have a shelf life. Epoxy will not harden if too old. I have never had this problem using plain old JB Weld.

BTW the over rated Rocket Poxy has a short shelf life I hear.

The curative/hardener may have a limited shelf life since most are amines or amides that decompose very slowly, giving a yellow to brown color (five minute epoxy curatives have a different chemistry and may have a shorter life). Even so, the reactivity doesn't necessarily change all that much. I have a quart of Fiber Glast curative given to me by my mentor. It's at least fifteen years old and is medium-brown, but still works a treat. The cure ratio printed on the rusty can still gives a nice glassy-hard product.

Most epoxy resins have very similar chemistry and the shelf life (for amateur use vs. industrial use) for most is on the order of a decade or even longer if stored tightly capped at room temperature. I have USComposites laminating epoxy that is at least seven years old. No change in properties that I've been able to see. No doubt there are specialized epoxies that should be used within a particular timeframe, but they're the exception, not the rule.

Best -- Terry

 

[OverTheTop]

I agree with the weighing of epoxy. Just make sure you get the right mass ratios as the densities of the components can differ.

 

[KilroySmith]

Now, 5 of the fillets and solid and smooth, but one is still sticky.

When mixed well, Epoxy should be a uniform material, which means (to me) that if one fillet cured well, then all of them should. ISTM that this is more likely a case of poor mixing than poor measuring.
I'm not going to argue with any of the recommendations given above - a scale and a measuring cup is truly the best approach - but in my experience even eyeballing the sizes of two blobs on an index card works fine as long as they're well mixed. Almost as good as weighing is doing it by volume - if you have a graduated measuring cup like these , pour in 5 ml of one component (the thinner one preferably), fill to the 10 ml line with the second component, and you're golden.
Mix'em by weight, mix'em by volume, mix'em by eye, but whatever you do, mix'em well.

Now, once you get into specialty epoxies, the measuring may need the precision of a scale. But anything you buy in a blister pack isn't likely to be very picky.

 

[Woody's Workshop]

Was it the last fillet you applied from the mix?
And did you use every bit you mixed up?
Did you mix it up in a cup or on a flat surface?
Typically, when everything turns out except that one thing like that...
You got down to the end of the mix and may have scraped some from the mixing surface that didn't get mixed up well.
I know it's wasteful, but mixing a little extra makes sure you get a good mix on what you need.

 

[billdz]

Was it the last fillet you applied from the mix?
And did you use every bit you mixed up?
Did you mix it up in a cup or on a flat surface?
Typically, when everything turns out except that one thing like that...
You got down to the end of the mix and may have scraped some from the mixing surface that didn't get mixed up well.
I know it's wasteful, but mixing a little extra makes sure you get a good mix on what you need.

It was not the last fillet, and the mix was in a cup.
The redo turned out fine, thanks to all for the advice.

 

[PayLoad]

40 years used the "Eyeball-it" method. Never once had an issue.

I use J-B Weld 5 minute epoxy on. absolutely. everything.

 

[G_T]

Measure by weight or by volume as appropriate for the particular epoxy system you are using, into a single container.

Mix till you think it is well mixed, including repeatedly scraping the sides and bottom of the container.

Transfer to a fresh container, and mix again with a new mixing implement. Now it is uniformly mixed.

Use this approach for any critical assembly operations.

As for things like "well cured" "works fine" etc, unless you have taken measurements or otherwise performed tests, all you are really saying is that you got a solid out of it. You are not likely getting the solid with the best properties you could have gotten out of it, if you are not going through the work up front - cleanliness, good measuring procedures, good mixing procedures, proper temperature range or cure cycle, etc. And many times it doesn't matter. But sometimes it does.

Gerald

 

[prfesser]

Measure by weight or by volume as appropriate for the particular epoxy system you are using, into a single container.

Mix till you think it is well mixed, including repeatedly scraping the sides and bottom of the container.

Transfer to a fresh container, and mix again with a new mixing implement. Now it is uniformly mixed.

Small but important addendum: most of the epoxy firms say to transfer to a second container without scraping the first one. Probably less critical for most rocketry, but if one is doing a clear epoxy tabletop, a single sticky spot will generate Many Bad Words.

Best -- Terry

 

[GlueckAuf]

I’ve used these reusable, accurate, American-made Mini Measure shot glasses, available at BB&B, to ensure a precision JB Weld slow cure mixture. Fill to half the needed volume of resin, add identical volume of hardener, then mix thoroughly until it’s all an even, dark grey color, scraping the glass sides, too.​

​

​

J B Weld’s slow cure variety is stronger than just about any of the clear, fast-curing epoxies I’ve used. It cleans up easily with 91% isopropyl rubbing alcohol, vs the clear types that always seem to leave a sticky residue. JB is also tolerant of inaccurate mixes when just a little dab’ll do ya.​

Good skies,
GlueckAuf

 

[Hobie1dog]

Measure by weight or by volume as appropriate for the particular epoxy system you are using, into a single container.

Mix till you think it is well mixed, including repeatedly scraping the sides and bottom of the container.

Transfer to a fresh container, and mix again with a new mixing implement. Now it is uniformly mixed.

Gerald

If I tried all of these steps with 5-minute epoxy it would already be set up.

 

[Antares JS]

This reminds me that one of the big advantages of West System is the mini pump set. Pump of resin, pump of hardener, done. No need to mess around with weighing.

 

[OverTheTop]

This reminds me that one of the big advantages of West System is the mini pump set. Pump of resin, pump of hardener, done. No need to mess around with weighing.

I never bothered with the pumps and do all West Systems by weight. That means if I only need a couple of millilitres I still get the mix right. Pumps are probably convenient for larger quantities though, although I have found weighing is not inconvenient IMHO.

 

[prfesser]

This reminds me that one of the big advantages of West System is the mini pump set. Pump of resin, pump of hardener, done. No need to mess around with weighing.

That's fine if a couple of ounces is needed, but for smaller HP and most modrocs, that's way too much. I rarely need more than a teaspoon of epoxy except when glassing, filling the tip of a nose cone, and the like. And if it's being mixed properly, now that inexpensive digital scales are available I spend a lot more time mixing a batch than I do weighing. Though I cheat I have large (50 mL) syringes to dispense resin and hardener.

 

[Antares JS]

That's fine if a couple of ounces is needed, but for smaller HP and most modrocs, that's way too much. I rarely need more than a teaspoon of epoxy except when glassing, filling the tip of a nose cone, and the like. And if it's being mixed properly, now that inexpensive digital scales are available I spend a lot more time mixing a batch than I do weighing. Though I cheat I have large (50 mL) syringes to dispense resin and hardener.

Ah, but in that case, you take a ruler to the pumps and mark them along their lengths at half a pump, quarter pump, etc. I myself use half a pump most of the time. Works great.

 

[Lord Rory Gin]

That's fine if a couple of ounces is needed, but for smaller HP and most modrocs, that's way too much. I rarely need more than a teaspoon of epoxy except when glassing, filling the tip of a nose cone, and the like. And if it's being mixed properly, now that inexpensive digital scales are available I spend a lot more time mixing a batch than I do weighing. Though I cheat I have large (50 mL) syringes to dispense resin and hardener.

I've transferred by Aeropoxy into smaller 8 oz squeeze bottles, then dispense into small plastic cups for mixing. A digital scale to 0.01 grams is what I use to measure the amounts. Your right, usually only a few grams at a time is needed to be mixed. When I used the 15min BSI, I learned to work quickly and efficiently. I was once caught off guard when I mixed up a larger quantity (20 g), it cooked off while I was still applying it and became unusable after 5 min! So I will never use larger quantity amounts of the quicker epoxys again.