Never intentionally alter the ratio of epoxy resin and hardener. Everything should be done to get an exact a ratio as possible, whether by volume or weight. Regarding mixing by volume, be cynical that the mixing cup graduations are correct. Using the same graduated mixing cups, my scale revealed they were imprecise. Usually in favor of too little hardener, poured second.
If the ratio is off slightly, it will still likely still cure to sandability, but it will not have all its full physical properties. If I screw up when mixing, like going way over on the hardener, I do not just add 2x as much extra resin to compensate, and continue. I pretty much curse and give up on that batch of resin, as it is not worth it if it does not cure properly, and these are smaller batches used on fins or repairs.
Enough to laminate a whole side of a board, one better be precise, accuate, and careful. I make lots of tiny batches of epoxy, and used to do so by volume. It is Much easier to get the ratio wrong when mixing small batches by volume.
When I have improperly mixed epoxy it remains fingernail denting soft, it sands weird, smells different, bonds poorer, flexes different, and when polished, it looks different in good light.
Epoxy strength and durability and workability are obviously compromised and inferior when mixed improperly, the degree to which it is affected depends on how off the ratio was, and no doubt some epoxy is more tolerant of imprecise ratios than others. But never think it is of no consequence and ‘just fine.’
‘Just fine’ is often in reality just barely above failure, in my opinion. Seems to be the phrase used by those intentionally doing things wrong with inferior products or by those who can’t be bothered to care about results and say 'close enough"
Epoxy can really punish those who say ‘close enough’.
I now mix Epoxy by weight to the 0.01 gram +/- 0.03(a drop of Apex hardener at 70f is about 0.05 grams) and my mixing stick is shaped to scrape both the sides and bottom of the cup at the same time, which significntly reduces the time to a complete mixture. All 5 sides of the mixing stick get scraped along the same side of the cup, more than once during mixing, and all 5 sides of the mixing stick are square, not rounded. Careful to not fling resin upwards near one’s eyes, when scraping its side on the inside of the mixing cup.
Scraping the sides of the mixing stick and cup early in the mixing process will reduce the time it takes to completely mix the resin and hardener. This is important as when the clock starts ticking once hardener hits resin, things can get rushed, and mistakes more easily made.
It is so not worth the potential poor results incurred from of mixing it improperly, that one should not apply any of their PE resin mixing habits to epoxy. It has to be a precise ratio, and mixed thoroughly within the mixing cup. Letting any drip down the outside of the cup when mixing can and will screw up the ratio to some degree, which is pretty much a NON factor with PE resin.
A strong light shined into the epoxy mixing cup one has stirred for well over a minute, can reveal swirls and whorls, indicating ethe epoxy is not thoroughly mixed, these swirls often cannot be seen without a strong light source shined into it. Doubters should try it before skoffing. I have excellent workshop lighting, and a 700 lumen headlamp on my forehead and some reading glasses reveals so much more than double 8 foot fluorescents 4 feet above does. No extra lighting available?.. take the cup into the sunlight to see the swirls and whorls in the mixing cup.
I have experience with few different epoxy brands. West, which I hated, System 3, Apex, and DMC. The System 3 general purpose resin is nothing special, likely well overpriced for what it is these days, but their clear coat resin is super slow and thin and my favorite for lamiating when time is not a factor, or for installing fin boxes in one step.
Their SB-112 resin is my favorite ‘fill’ or ‘final’ coat resin. I did not even know what a fisheye/orangepeel was until I started using different epoxy resins and started to have to take all sorts of extra steps to prevent them. Never had any visible blush or humidity related issues with sb-112 either. I cant say it laid down perfectly flat though, but no fisheyes, brush strokes could still be seen especially when overworking it. They advertise it as a tie-coat, in that polyester resin will bond to and cure properly to it. System3 sb-112 is not cheap epoxy and is an old formulation with potentially very old stock being sold online, but it is still my favorite, of the epoxies I have so far tried.
I’ve not tried System3’s Silvertip epoxy, which is their latest formulation. They kind of market it as being well superior to, and replacing most all their other epoxies. Likely immaterial to surfboard production though as it is likely 2x the price of RR and other surfboard specific epoxy resins.
Older epoxy resin might get all crystalline in the bottle when exposed to cool/cold storage temperatures. I had some Apex epoxy resin pour normally, but then I noticed a it had grown clear branches within the bottle when it was tipped, only noticed as its weight when tipped, did not respond as anticipated. There was a crystalline bush stuck to teh bottom several inches up. Heat the offending resin in its bottle to 120f for a while. I use a 5 gallon bucket, and raise the water temp to 120f and occassionaly lift it out and swirl the bottle around. System 3 resins will go from white back to clear doing this. The Apex epoxy worked normally afterward such a treatment dissolving the crystalline bush.
I bring this up as the Apex resin bottle gave no obvious Visible clues it required this treatment, until I poured it. The clear bottle showed it was still blue and completely normal looking. The latest batch of Apex resin I got comes in a white plastic bottle and would be even harder to notice if it had crystallized within due to time and low storage temps.
It is not worth the hassle when epoxy does not cure properly, so one should always insure the resin is still fully liquid throughout the whole bottle, that the ratio of resin to hardener is as precise as possible, whether by volume or weight, and that it is mixed thoroughly within the mixing cup.
These are all things PE resin is not really subject to, or all that concerned with. Such an attitude with epoxy will likely have the applier bad mouth working with epoxy resin, when the fault lies at the applier’s feet.
I very much regret the time and material wasted when mixing small batches of epoxy by volume. I really wish I got the scale much sooner, as not only can one mix tiny batches precisely, they can also mix muxh smaller portions without fear. I used to have to mix no less than 15ML to insure it cured ‘good enough’ when I only needed 1/3 that amount for the task at hand.
Now I can make sub 6ML batches precisely, without worry that it will cure properly, and waste significantly less epoxy doing so, and reuse the same mixing cup over and over.
Digital scales are cheap, and will often pay for themselves in that one can use just about any mixing cup they want, rather than the more expensive, harder to procure ones, with trusted graduations on them. I pour resin, take the grams, multiply them by 0.44, hit the tare button on the scale, and pour that much hardener.
The epoxies I’ve worked with vary from 100:43 to 100:45. Mixing small batches of epoxy by volume required a new clean cup on a level table and leaning over and trying to account for meniscus at different epoxy temperatures.
Get a scale and a dollar store calculator with big buttons, its faster and easier, especially when smaller batches are desired. It is a huge regret that I did not get a scale sooner. I’ve been finding some fins I made nearly 20 years ago for fcs1 tabs, when small batches were required to fillout the tabs or depressions in the foil, to have layer separation and other strength hardness and finishing issues, all because I thought mixing small batches by volume was ‘good enough’.
Don’t take chances with epoxy. Mix it precisely to ratio, and thouroughly within the mixing cup, and mixing by weight will yield superior more consistent results for most users. The digital scale will pay for itself!
