Epoxy filler coat question.

Im using L235 epoxy resin ( thats all thats on the bottle. ) and i was just wondering what i can put in it to use it as a filler coat.

If i just use the resin alone it stays sticky and is impossible to sand.


Would there be a generic product for this purpose?

Hi -

Don’t know of a specific product to achieve a tack free surface. Generally, epoxies cures tack free on their own if mixed in correct ratios and used within specific temperature ranges. Some take a few days to fully cure. Maybe it is one that benefits from a post cure heat treatment? How are you determining your proporions? There are some oddball mix ratios out there. If you are sure you have the proportions right, maybe try a test panel post cure with some heat?

Hey John,

Ive tried a few different ways to cure the epoxy. Firstly, i always weight my mixes exactly to instructions.

Secondly, i heat the resin to aprox 40 - 45 degrees c . Thirdly, i glass the board on a falling temp from about 45c . After it cures or gels i usually bake it at 50c for 3 hours.

Its funny because sometimes it stays sticky and other times i can sand an entire board with one disc. Im quite confused as to what causes this.

I’d agree with John, it sounds like the resin is not cured propery.

You will definitely need more time than 3 hours after the resin has gelled, even at that elevated temperature.

Even if you waited over night then postcured, 3 hours still isn’t much.

Usually waiting over night for the resin to set by itself, then postucuring at 40C for 8 hours is the way to go.

There might be something else going on of course, but try giving it some more heat, see if that helps things!

Epoxy clogging the sandpaper is caused by the heat generated softening the epoxy, and causing it to smear.

If the epoxy is cured enough, it won’t soften with the heat, and should powder off when you sand it, rather than smearing.

Let us know if that helps!

I should say here that even though ive baked the board for 3 hours, i usually wait days or re-bake the board several times before trying to sand it with no results.

When you use poly filler, it powder’s off. No such luck with epoxy…

Fair enough, there must be something else happening… my guess is blush.

When its humid you can get problems with the resin blushing, moisture from the air is absorbed by the resin, and sits on the surface of the cured epoxy as a slimy film.

It clogs your sandpaper like nobodies business!

Wash you board down with water, scrub really well with a scotch bright/green scouring pad.

Another trick I use is to then go over the board quickly with a piece of scrap 240 grit, wetsanding, just roughing up the surface.

Doesn’t have to be proper wetsanding paper, just anything you have lying around.

This sorta breaks the surface of the epoxy, and when you dry sand it like usual, it sands waaaaaaay better.

All the blush is gone for sure then!

Blush doesn’t always occur, it depends on the humidity of the day, how long the resin takes to dry, temperature etc etc, so that would totally explain why sometimes you have problems and sometimes not.

If your resin is fully cured and well washed with water, you should have more luck.

Thank you for taking the time to go over that for me. I have a sup paddle glassed and ready to sand so ill do what you say and let you know how it goes.

IMG_0410.jpg picture by belinzee

Not sure if Add F will work with other epoxies but it sure makes RR a damn sight easier to sand. Speedneedle suggested wax in styrene once as well but I never tried it.

since you are weighing your ratio, make sure that the ratio you are using is in reference to by weight. by weight vs by volume are often different ratios.

Yep, wax in styrene or xylene makes the filler coat flow out well. no fisheyes and easier to sand.

I like to let my epoxy fill coats setup enough to be handled them bake them at 50 degrees C for between 3 to 5hrs.

sands just like poly.

I’ve been meaning to try the xylene and wax trick.

What kind of wax do you use?

There’s a million different types of paraffin…

I’ve got some wax in styrene, you say that works just as well?

Additive F seems pretty seriously expensive over here, and one supplier tells me I can only have some if I buy RR from them at their premium prices.

I’ve got a nice xylene based epoxy thinner, just need to figure out which wax to put in it?

Can’t find a deffinitive answer in the archives.

Although I find sanding epoxy no problem already, it can’t hurt to make it even easier eh.


I use Paraffin preserving wax, it comes in a solid block in a container. they use it to seal jars when you preserve fruit and stuff I think. You can get it from Coles or woolworths supermarkets. or the local hardware. I usually make up about 500ml at a time. sit the xylene or styrene in a tub of boiling water and add the measured amount of wax abit at a time until dissolved.

Wax in styrene works as well.

the people who make the epoxies tell you not to add anything to their resin as it may compromise the properties of the resin, but seeing as it is only the filler coat there shouldn’t be a problem with strength, well I haven’t notice any difference in the strength of the finished surfboard.

Tried a few filler coats without it just to see the difference in sanding etc. and it was noticeable. with the wax added you get a finish that looks like a gloss coat. and sands like a poly filler coat.

Thanks a bunch man, i’ll give that a try for sure.

How do you figure out how much wax to add? grams per litre?

I agree, the filler coat is not critical to overall strength.

As long as it doesn’t make it super weak so it chips off, or anything else bad happens - I doubt it would be affected that much.

A few people here make their own. The mix that has worked the best so far is a 10-1 ratio, xylene to wax. If you have a scale you can figure it out. Also be careful with any heat source and the xylene.

If it is warm enough then the paraffin will disolve on it’s own in the xylene.

I’ve been using 5% wax in the xylene and adding 3-5% of that mix to the epoxy.

As DMP said no live flame or heat source to melt the wax, a hot water bath is sufficient in cooler weather.

Had no problems with chipping or flaking, sticks just the same as unwaxed epoxy.

Give it shot. Let us know if you find any difference.

Cheers a bunch guys, i’ll give that a shot

I haven't tried to make my own yet but for the regular Resin Research Additive F I'll use 2cc's to every ounce of hardener for a sanding coat. I was using a syringe that had cc marks that I got at a pet supply store, but the xylene eats the rubber plunger on the syringe making it sticky. I noticed that a childs medicine dropper that looks like a miniature turkey baster and the small medicine dispensers for childrens tylenol are marked in cc's. The mini turkey baster is nice because it is easy to suck small amounts of the Additive F out of the can and it doesn't clog or get sticky.

I've also gotten way better results with the Additive F after putting the liquid in a glass mason jar. Now I can see when the wax starts to precipitate in the xylene at temps. as high as 70 degrees F. I'll put the jar in my pocket for a few minutes or warm it up in a bath of warm water to make it clear if it's cold. Using the metal can hides the solution making it hard to tell if it's cloudy. I've found that by having the mixture completely clear you can get way better results for a sanding coat especially over darker colors.

I was also trying to be responsible by not generating excessive waste by having my can of Additive F re-filled instead of throwing the old one away when it was empty. The end result was the can filled up with all kinds of garbage, dust, rust scale from the inside of the can, and stuff I couldn't identify and I really believe it was screwing with my sanding coats. The glass jar has been an easy fix to some annoying problems.

I use a graduated cylinder to measure my Add F and X-55. I use calibrated poly beakers for mixing - accurate to the mL. Both are available at any lab supply company. They’re high quality, so they’re easily cleaned and reused - I simply pop the cured resin out of the beakers, and wipe them with a clean, dry shop towel. No scales, no sovents, minimal waste. Way worth the investment.