Measuring Epoxy: Warning

I recently completed a travel board for a friend. I used RR epoxy and I love the stuff, but the bottom on the board was SOFT! It was easy to make a permanent dent with medium thumb pressure. All that work, and I end up with a dent magnet.

I had measured the epoxy exactly using Home Depot plastic measuring cups. Tonight I discovered the problem: the cups sold by HD are not accurate. The result of me trusting the graduations on the side was I ended up with too little hardener.

I did a check with some measuring cups from an auto paint supply and they were dead on. I’ll use them in the future.

The moral of the story, as I slap myself on the forehead, is to check the accuracy of any measuring cup. RR epoxy needs to be a 2:1 ratio, on the nose.

I just wanted to pass this on so someone else can avoid the frustrations I just went through.



not like PE resin yes.

try measuring by weight? digital scales are cheap and can be used for many other things other than measuring resin. I only measure my 2:1 epoxy from Fiberglass hawaii by weight. I got my scale off of ebay…2o bucks plus shipping. My scale can weigh items up to 35lbs. real easy! give da bugga one try

measuring by weight seems like it would be the more reliable solution. But are the densities of the RR resin and hardener the same? They say to measure by volume at 2:1 on their website. I’m wondering if measuring by weight might be problematic if you’re dealing with unequal densities.



measuring by weight seems like it would be the more reliable solution. But are the densities of the RR resin and hardener the same? They say to measure by volume at 2:1 on their website. I’m wondering if measuring by weight might be problematic if you’re dealing with unequal densities.


The densities of the hardener and resin arent the same. The ratio for RR is 45:100…the ratios for other resin systems vary slightly also

I’d go back to HD and tell them they need to cover your cost of that board due to their faulty product…It’s worth a shot.

Doug, someone here turned me on to this web site I think (

I bought a digital scale for about 30 bucks. Saves time, saves headache, makes stronger boards, keeps the positive vibe, avoids crabbing at the wife and kids, avoids kicking the dog. Lot’s of spin off benefits.

I got the screw on hand pumps from fiberglass supply and just use them to feed the material into the cup on the scale. The scale zeros out the weight of the cup or bucket and any hard resin from the time before.

And, here is the best part, I can measure out a single oz to do ding repairs very accurately.

BTW I bench marked those graduated paper cups from fiberglass supply on my scale and they were not even close. Scary.

Thanks for the link Greg. What’s the capacity of the scale you got from them? The ones I’m finding at a low price only measure up to 500g, (1.1 lbs.) That’s about one pint of liquid tops. It doesn’t seem like that would be enough for a whole batch of epoxy.



Edit: Oh, just found the one you must have been talking about. It goes up to ~11lbs, and is accurate to 1g. Great site, they have free shipping too.

At this point I hope Greg Loehr chimes in. His website does recommend measurement by volume. If, as stated, RR epoxy is 45:100 ratio by weight, you would end up with a little extra hardener in the mix. I think that would be better than not enough.

As we speak, I’m preparing to strip the bottom of the board and reglass. My head hurts.

“The best way to avoid mistakes is experience. The way to get experience is by making mistakes.”



… BTW I bench marked those graduated paper cups from fiberglass supply on my scale and they were not even close. Scary.

Really? That’s scary indeed. I’ve been using the 1oz cups to measure out as little as 1/4 + 1/2 of an ounce and the 16oz cups to measure lam/hotcoat batches without any problems. Knock on wood. How much are they off?

Greg Loehr, is there a consistent margin of error that the resin mixture can be off by, and still harden/cure right?

I feel your pain, Doug.

My (near) disaster (not as bad as yours) was the time I mearsured up a batch of hardener, then one half of the resin (intending to do it with two cups since the cup was small) and then… the fedex guy showed up. You think he was in a hurry? You think I remembered I only had half the needed resin? You think that little bucket got hot? You think some people can’t remember what they had for breakfast? Right.

That bucket o’ resin would have melted my EPS blank. Would have been a great pic. But it burned my hand so bad I dropped it. Good thing it only went all over half of the garage floor instead of the whole DAMN THING! Excuse me. Flash backs are hell.

Hey Ozzy,

I’ve “eye-balled” RR before and it seems to work out fine. I’ve only done this in small batches for ding repairs and such. There seems to be a margin of error that seems to be pretty wide…just from my experiences though.


BTW another reason for soft resin is inadequate post curing…

RR should encourage measurement by weight it is much more accurate.

Thanks Daklaw. I wonder if Greg has a specific (percentage) number for the margin of mixture error.

I don’t agree with measuring by weight as the default. I’m sure it works fine, just like eyeballing graduated cups. But if the two resin components have a different specific gravity, which they do, that would throw off measuring by weight and may be pushing the margin of error envelope more than mixing by volume. Maybe not, I’m not sure. I do know that Greg has developed, tested, and produced a great product rivaled by none. And if he recomends mixing by volume, then I’m going to mix by volume.

Check the MSDS. Hardener has a specific gravity of 0.98 and epoxy is 1.10 (water is 1.0 which all other specific gravities {weights} key off of.) Unless you’re factoring in the different weights, I don’t see it being more accurate. Easier maybe, but not more accurate. – i.e. there is a 12%(?) difference in the weights of hardener and epoxy. Is this correct Greg?

I normally tell everyone to measure by weight since like others have found those measuring cups (depending) can give inconsistant results. Since ive switched to measuring by weight its been consistant.

For larger production enviorments there are a few products that can not only pre measure the epoxy but also mix and output it right into a bucket. Off the top of my head its called “goo grinder” from its pricey though

It’s a simple task for me.

just use the same cup for both the resin and hardener. I put 1/3 of the amount I’ll need in the cup and mark the level. I pour this into my mixing cup and put a little additive ‘f’ into it if I’m doing a fill coat. Then I just fill my measuring cup up twice to the mark. I don’t have to worry about the exact measurement of the total. The measurements have to be right.

How hard can it be?

No Worries, Rich

P.S. I almost always mix a little less than I need and finish the project with a small batch I can baby sit with.

have not bought mine yet, but if you go to ebay and search on “lb digital scale” you can scan the results for scales in the 35-50+ lb range. Looks like after shipping your looking at about $20-30 for the 35/36 lb variety.

looks like accurate to about .02 oz depending on the model.

b.t.w. for vac bagging I like the idea of measure by weight since after you cut your glass you weigh it and then at tells you how much resin you want. (i.e. 1:1 ratio)

The 45:100… that was the by weight ratio Greg answered me with when I asked at the 1st Ceritos College event. That would be for RR epoxy.

By weight. Always.

1 - .45 for RR

1 - .4 for Aluzine

Always by weight. Always.

Right on the money Halcyon. But sometimes we have to find reasons to go buy another tool. {Insert primal grunts here} I know I do. lol

I’m a 28 hour drive from Cerritos and I didn’t know Greg gave a specific ratio for measuring by weight that is different than measuring by volume. Spending too much time in da shop I guess. And actually, going by the specific gravity, shouldn’t the ratio be 44:100?

Either way, it doesn’t matter to me. Just curious. I am never going to mix by weight. Don’t need to.


fiberglass hawaii brand 2:1 epoxy (100:40 by weight) and they have free charts for part a,part b and sanding additive.

I took two identical measuring cups to weigh 60ml of each (part a and part b). The final calculation is very close to chart given by FGH.

check ebay for digital scales, I won one for 20 bucks plus shipping. I have calibrated weigh bars from college that I use to calibrated/verify my scale. And my scale can weigh up to 35 Lbs.

Here’s what Mr.J uses to do his stuff…

Sanity Test

where is this guy anyways??? he had some really cool stuff going on…way ahead of the curve…

hope that helps…