Epoxy yellowing ding repair

Hi guys, I’ve been fixing epoxy dings with this stuff i get at Mitches Surfshop.  It’s super strong, sands out clean, but turns yellow in some spots.  Not sure what the deal is, I mix it in a clean cup with a popsical stick.  Nothing could have contaminated it.  Kinda bummed out because this board is a super white tomo.  Any advice?  Maybe there’s some nuance I’m missing.  Thanks. 




The popsical stick could be transferring the color of wood to the epoxy.

 

The ‘some parts’ being yellow might indicate incomplete mixing

Small batches of epoxy resin are so much better measured by weight rather than volume.

 

My mixing sticks are almost always a hardwood, already presaturated with epoxy and hardened.

Their corners are sharp, and the bottom fits the bottom of the mixing cup exactly, a round popsical stick cannot get into the cups corners.  The pretuned mixing stick reduced the time to achieve thorough mixing, and prevents several issues that those indoctrinated with poly, can have when using epoxy.

 

Epoxy ratio is never to be willfully altered, and all steps taken to get is as precise as possible.  If 10% out it might still cure and sand OK, but it will not have the same bond strength nor overall strength has it been mixed by weight precisely, and then thoroughly mixed in the mixing cup.

 

Make sure to scrape the mixing stick several times on the flat edge of the mixing cup, preferably the same part of the mixing cup, and use a strong light and perhaps some eye magnification to insure there are no ‘swirlies’

Epoxy is unforgiving of imprecision so it behooves one to eliminate the variables which contribute to imprecision.  Get a digital scale.

 

One drop of Apex hardener is about 0.04 grams, at 75f, and I aim for accuracy less than 0.02 grams.

 

The apex epoxy is 100:45 ratio by weight.  

 

So I pour the resin, then multiply that  resin weight by 0.45.  I hit the tare button then pour that much hardener into center of mixing cup, if I go over I can lift some out  of center top of cup with the mixing stick to attempt the perfect ratio to 0.01 of a gram.

 

My Biggest regret ever regarding working with epoxy, was thinking I could mix small batches by volume with any degree of accuracy.  The digital scale revealed the graduations on my most trusted small mixing cups, to be unreliable at best, nevermind the meniscus.

Since I switched to the digital scale there is never any question of a perfect cure.  Each batch passes the fingernail test at 24 hours. Imperfect batches almost always failed, yet still sanded fine and appeared to be more than adequate, for a while anyway.

 

Also when mixing A and B, any which drips down the side of the  mixing cup, screws the ratio, and potentially the cure, depending on how quickly the drip down side of cup occurred when mixing.

 

a ‘good enough’ approach to epoxy, often yields results nowhere close.

Eliminate the variables/ human error factor.  Mix thoroughly,  the proper ratios,by weight, and don’t look back.

 

Make your mixing stick fit the cup, and if a new mixing stick, perhaps soak it with acetone and wipe it repeatedly first, so it cannot transfer any wood color to the epoxy.

+1 for the lightly off mix. As said you need to be accurate for your mix, harder with small one then you must thoroughly mix it, multi scrap everywhere around your mixing container and avoid to use mix sticking on surface, that’s why sold use double potting, a tiny bit off hardener excess and it yellow.

Sanded laminate fiber is also a bit more yellow even more with those super white with brightener, resin. 

Thanks for the comments. So more details on how I mix it… I use a syringe, take the plunger out, pour in the epoxy in the top to the 2 marker, shoot it in the cup, and then do the same with the hardener, pour to the 1 marker, shoot it into the cup.  The hardener is yellow, not mixing better could be the culprit for sure.   I used to mix for 2 min.  But I’m also working with poly so got a big lazy mixing I think.   Both the epoxy and hardener would have a meniscus so they cancel out, I feel like it’s quite accurate with thr syringe and I’m very efficient with the material measuring this way.  The cure is always spot on.  Thanks for the tips I’ll consider them next time and advise back.  


Buy Epoxy that contains a “Brightner”.  It will have a blue or purple hue to it in the container.  Your Tomo was glassed with Epoxy that contains a brightner.  What you are using is a stock Epoxy with no brightner.  Susceptible to UV yellowing.  Rev-Chem pours off this resin into Composit Resource containers at their warehouse in Bloomington, California.  Composite Resource is their small quantity retail brand.  They sell an Epozy with UV Brightner.  If the Resin you are using sits on the shelf in a store or warehouse, it will turn brown before it’s even sold.  The resin you bought works fine, but is UV sensitive.  If you want cosmetics go to an Eoxy with UV Brightner.   The Epoxy you are using is comparable to Resin Research 2000/2100.    PS -   It’s really not a measurement problem.  It is a resin problem.   Composite Resource’s UV Epoxy will cure bright white.

I had same problems with RR ultra not before with CE. No more problem since i use a local brand with a purple UV blocker. I don’t use their brightener that allow more white than white up to it start to yellow… 

Funny thing about swaylocks.  

A guy asks a question,  gets a good answer,  and then replies with how he’s doing it his way, while ignoring the answer.  Don’t feel bad.  Everyone does it.  It must be human nature. 

We’ve tried the syringe thing too.  It doesn’t measure accurately.  Maybe the different viscosies of the two parts?  Part A is thicker, so gets hung up in the spout?

Anyway, wrcsixeight is right.  Gram digital scale.  Dixie cup.  Popsicle stick with the round end cut off.  Never mix less than 10 grams Part A and .4 grams part B ( at least that’s the ratio of the epoxy I use) 

Thanks pal, you have a link to where I can buy that epoxy with the brightener?

Sorry if that came out wrong just trying to shed more light on my mixing technique , I’m not married to it, I agree I should buy a digital scale

Got a link to where I can buy some better stuff?

As I said;  I know Tomo uses a resin with a Brightner.  Which one I don’t know.  You can buy the Composite Resource with a Brightner in it probably thru Mitch.  If it’s not on the shelf, he may be able to get it for you.  Rev-Chem is in San Diego County for deliveries every week.  Otherwise go to Resin Research’s website and read up on their resins.  I sell a lot of 2000 Clear.  Looks good and clear over painted and clear EPS boards.  Pretty good UV resistance.  They also sell CE Blue and Ultra Plus with Optical Brightner.  Foam EZ sells RR.

You can get little Resin Research CE/2100F ding repair bottles here:  https://www.foamez.com/product/e-z-epoxy-6oz-kit/

I use small measuring cups that show markings in drams when doing small batches.  It’s better to mix up more than you need, and have the mix ratio perfect, than to wing it with a really small batch.