# Measuring Epoxy by Mass not volume - Caution

So I’m day dreaming about the board I’m gonna build. I’m stupid enough to try a swirl on my first go 'round and that got me thinking about a bunch of small cups of colored epoxy and hardener. Gotta keep proportions correct. Me thinks, okay, I’ll use a precise scale and do them by mass. Simple enough.

Hmmm…the proportions are 2:1 by volume. What if the resin and hardener are different densities? That could screw me up if I’m weighing them. I’ve heard that you’ve got to keep the proportions accurate or you’ll be in trouble. I don’t want trouble.

I checked the MSDS from RR for specific gravity and sho’nuff the resin is about 12 % more dense than the hardener. I don’t know how critical a 12 % volume error is but it don’t want to chance it.

FYI - The SpG of the resin is 1.10 and the hardener is 0.98. Mulitiply your desired volume in CCs by those numers to get the grams you need to weigh up.

In other words.

Resin: 1000 cc x 1.10 = 1100 g

Hardener: 500 cc x 0.98 = 490 g

This will maintain your 2:1 ratio by volume.

This may have been addressed before but I was just thinking about it and thought it might be useful.

thanks, Ryan!

this issue has been addressed before, but you laid things out in a very simple and straightforward manner (which had not yet been done, to the best of my knowledge).

epoxy users should print that out and post it up above their scale.

I know what you mean,

I do my repairs with RR now and small amounts are a pian to mix up, 15ml of resin goes a long way, hate wasting resin but won’t risk mixing less.

If your doing a swirl, epoxy has a longer working time than poly so why not just mix up you total batch then seperate and add the colours to the pre mixed resin.

If your worried about time invite a friend over for an extra pair of hands to help mix in the colours.

Ryan,

They (RR) recommend a ratio of 100:45 (Resin to Hardener) when mixing by weight. Like you suggested this is due to the different specific gravities of each component.

Dave_D

Thanks Ryan,

For small batches, I use a small digital scale from HF for \$13. You can zero it out (tare) with your small cup on it and mix accordingly. Measures up to 500g

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93543

Thanks, bud…

We’ve been doing a few swirls, and found that the cups that have the yellow epoxy pigment we use (sorry, I don’t have the brand name with me right now)kicks faster than the other pigmented cups. Even orange, green, etc… anything that has the yellow in it at all… kicks faster. Straight yellow kicks so fast it starts to pur out like stringy snot.

Anyone???

Woody,

We’re on the same page. I’ve still got to get the stringers in and shape the thing but I plan something like that. I’ll definitely get some extra hands in there. I’ll also mix a larger batch of the main color then divide it for the smaller pours. I’m thinking about disposible carboard sheets to lift the laps so the swirl can carry over the rails. That will defintely require two people. I’ve got time to work out those details though.

Thanks.

Quote:

They (RR) recommend a ratio of 100:45 (Resin to Hardener) when mixing by weight.

Dave, That makes sense. 0.98/(1.10*2)=0.45. I figured people were on top of this but I was thinking of it so I thought it worth the post. I think if I’m gonna weigh it, I’d rather use the formula instead of converting the ratios though. It’s just the way my head works. Desired CCs x SpG = grams to equal CCs.

30 cc x 1.10 = 33.0 g

15 cc x 0.98 = 14.7 g

Cool. Many ways to reach the same result. We’ll see. I haven’t had to do yet. I may find volume works fine for my purposes.

Has Greg L. ever stated his reasons on Swaylocks why Resin Research does not also add measurement by weight to the mixing instructions for their resins?

yes. he did a post where he explaned it… basically he didn’t want to frighten simple minded surfboard builders who were already so freaked out about switching to something rather then poly… imagine the backlash he would have faced… “your epoxy is already so expensive and now you want me to go out and buy a scale for \$13!!!” he was trying to keep it as simple as possible to get people to transation… the scale actually makes it so easier in my opinion… for small batches espically…

I look foward to seeing the pics.

If you do the swirl on the deck you can do it as a patch inside the lap of ther first layer so no need to worry about the rails.

If you do the swirl on the bottom splash the rails with the seperate accents before you mix them for the main swirl.

Ryan,

What I did was make an Excel Spreadsheet with the numbers for from about 10 grams to 400 grams and have it on the wall over my scale. I first weigh the resin and then add hardener to get the final weight. I even put a column in there for how much it is in ounces by volume when you need to figure how much to use for whatever job i.e. hotcoating, ding repair, fin box etc.

Dave_D

You guys are putting too much science into the RR Epoxy, just measure it out in any old way just do 2 to 1 it really doesn’t have to be super accurate. I mean if you miss it by 2.3 to .07 it will still be fine. The most important factor is the temp and weather. I just use dixie cups, 2 small cup fulls to 1 small cup full, stir it up, and pour it on. Our if you need a lot, 2 slurpee cups to 1 slurpee cup.

Quote:

I do my repairs with RR now and small amounts are a pian to mix up, 15ml of resin goes a long way, hate wasting resin but won’t risk mixing less.

If you use suringes(one for hardener and one for the resin, don’t mix them) it’s easy to mix really small batches. I find my scale is useless for anything less than 50 grams with a 5gram granularity.

regards,

Håvard

Haavard,

I have access to scales that measure to 0.01g so accuracy won’t be a problem.

That fact that I have access to those scales probably tells you a little about why I over think these things like Resinhead said.

Dave,

Good idea. Knowing me, I’ll probably make one of those too when it comes time to do it.

I bought two stainless steel hand soap pumps from Walmart, one for resin and one for hardner, they work great for small patches. Just have to give the resin time to flow back into the pump after each stroke.

NJ

Alot of yellow tints and colourings are based on aniline dyes (they’re amines), so yes, they’ll kick the resin a little faster than a non amine based dye. Further, most of the anilins aren’t colorfast, so they’ll lighten up or go brown with prolonged exposure to light. If you’re actually doing an opaque or transluscent “pigment”, rather than a tint, you’ll be fine to switch to an iron based colorant, which, as an added bonus, will give you a colourfast laminate. If it’s tints you’re after, then sadly, you’ll largely be stuck with the anilines. There are a few yellow quinacridones you could use that shouldn’t affect cure rate, but other than sourcing the raw pigment and grinding your own, I don’t know of a source for these.

I’ve had great results buying pigments here (http://www.danielsmith.com/cgi-bin/sgsh0113.exe?FNM=00&CATG=INFO+CAAA&UID=2007021611082775) and formulating my own.

HTH

If you guys want to weigh your resin that’s fine with me. You can buy these small buckets at the autobody Paint supply stores. See the ratios at the top where I put the X. We want 2:1 mix. About 16 oz total.

Follow the collum from the 2 down. Next fill your first part to the 5. (resin)

Now follow the collum from the 1 down and fill your second part to the number 5. (hardener)

That’s right just pour it right on top of the first pour. Bingo… perfect 2:1 mix

Yes they make smaller cups.

Yes this is the way Greg Loher mixed resin at the Cerritos college event.

Yes someone will post on Swaylocks and tell us that those cups are not accurate.

If it doesn’t make sense talk to the guy behind the counter or go to a place that paints cars and see if the painter will explain it.

Or do what Resin head says

Or keep weighing your resin…sounds like you guys have it figured out

I burnt my finger tips one time picking up a three ounce cup of epoxy resin that sat too long

Have fun!

Ray

That’s what I use… almost impossible to make a mistake.

I also get graduated plastic beakers from a science supply store that measure in mls… all sizes… all really accurate… all reusable…