View Full Version : Do you thin your epoxy coating?

03-31-2013, 11:30 AM
Do you normally thin the epoxy prior to applying a preservative coating on wood? If so, what do you use as a thinner and in what ratio?

Ian McColgin
03-31-2013, 11:40 AM
I'm one of those who is not an epoxy chemist and take no solace from others who are not, nor from propriataries with a vested interest, who claim that any thinners will do. I have also tried three propriatary sealer/rot chaser things and four propriatary epoxies with the thinners they recommend. After all that, when I want to seal and when I want to seal some wood after getting all the rot I could out, I'm sticking with CPES.

I don't know that it's the best in all possible situations but I do know that it's worked better for me every time I try.

03-31-2013, 12:14 PM
I'd not thin it. I would use formulated low viscosity like West's 207 hardener. Or "Git Rot" types of epoxy.

P.I. Stazzer-Newt
03-31-2013, 12:32 PM
No. act of an idiot.

Todd Bradshaw
03-31-2013, 12:34 PM
Absolutely not. If you want it to act like epoxy, you should not mess with it. This is not to say that there is no use for thinned epoxy at times as a primer for other coatings, but if you are seriously looking for the sealing and strength characteristics that epoxy resin has to offer, be aware that thinning it with evaporating thinners substantially reduces damned near all of them. Also be aware that the whole "deep penetration" thing is pretty much a crock. Real epoxy resin does not need, or rely on, deep penetration to do its work.

03-31-2013, 12:52 PM
I have used thined epoxy. It penetrates the wood and does not run much, unlike un-thined epoxy. Unthined epoxy runs terribly. I try to use epoxy on level surface as much as possible.
We have a lot of CPES fans on this site. CPES will fry your brain. I have never used CPES, but I will on the next good opportunity.
I wonder if the "No Thinner" crowd never use thinner in paint ??
I dry sail and store my boat on a trailer so the concern for rot is not very important to me.
Progrssive Epoxy Polymers website compares solvents, penetration, etc. I use anything that says it will mix with epoxy.

Ian McColgin
03-31-2013, 12:57 PM
Especially if you're into knowledgable mixing, it's hard to beat the variety of stuff and advice at Progressive.

CPES has been much covered. Just remember that the fumes can kill a rock. Read the directions and be safe.


wizbang 13
03-31-2013, 01:10 PM
If it is wet longer , common sense, not science, says it will "penetrate " a little deeper.
So, super slow tropic hardener, 207, will stay wet longer. I sometimes use it as a first coat on wood for that reason.
I use CPES , when called for.
I do NOT add acetone, alcohol, ketone , or what not to epoxy.

03-31-2013, 02:00 PM
I mostly use epoxy as a glue, not a coating. There are better products for coating purposes.

Todd Bradshaw
03-31-2013, 03:56 PM
I wonder if the "No Thinner" crowd never use thinner in paint ??

Yep, but that's apples and oranges and a very different situation. Adding thinner to paint does not instantly make something that was 100% solids into something containing evaporating thinners and as such, dramatically reduce its ability to function as a moisture barrier - or reduce its strength and adhesion. Paint already contains evaporating thinners and once you're done thinning it, it's still paint. The same can't really be said for epoxy resin. What you create when you thin it is basically a different product with different uses and characteristics.

As to epoxy running on non flat surfaces, it is a matter of learning technique, as well as understanding that epoxy does not brush or roll on in a "finished" state like paint or varnish. Even so, any time you get runs or sags in any material it's a clear indication that you are putting it on too thick, and the quality of the work will improve and need a lot less fairing if you apply thinner coats.

Working well with epoxy takes some practice and some studying. There is plenty of application and useage information out there for free, but unfortunately, most beginning builders never bother to look for it and read it.

There is one good discussion with test results here:

click on "Epoxy Techniques & Materials "
on the page that opens, scroll about half-way down the page to the article on "Thinning WEST Epoxy".

03-31-2013, 04:30 PM
I never thin epoxy. Also the rottenest boat i ever had the misfortune to work on (a 40ft ex admiralty diagonal planked motorlaunch conversion called the Green Parrot) had been treated extensively with GitRot. The rot didnt git at all, in fact it was a miracle that the boat was actually in one piece well sort of!

Bob Cleek
03-31-2013, 05:06 PM
While I have experimented a bit with "thinned epoxy" using acetone to "tack" the first layer when laying Dynel over a CPES'd surface without any apparent ill effects (full strength epoxy was laid over that immediately to fill the weave), I would not recommend it today, given the availability of "tropical" slow-cure hardeners. In fact, I don't consider epoxy resin adhesive to be a proper "coating" in any event. It is a resin for use as an adhesive to hold down fabric, create a structural matrix (carbon fibre) or glue pieces together. There are all sorts of epoxy based coatings that are expressly formulated for use as sealers and coatings intended to be thinned if need be, resin is not. (e.g. CPES and epoxy paints)

03-31-2013, 07:44 PM
I've used both. Have thinned epoxy with Xylene and used CPES. I think CPES penetrates better and I think it cures better. I don't slather it on everything but will use it sometimes for a localized issue.

03-31-2013, 08:21 PM
What about just warming the work area?

Bob Cleek
04-01-2013, 01:24 PM
What about just warming the work area?

Heat accellerates the curing process, sometimes radically.

Todd Bradshaw
04-01-2013, 04:41 PM
True, but it can increase penetration without changing the physical characteristics of the epoxy (read the Gougeon article).

04-02-2013, 07:41 AM
If you thin epoxy, the thinner will evapurate during the curing process. That will form pores in the coating, so you don't have the main benefit of a epoxy sealing - the poreless sealing for best watertight coating. Therefore never thin epoxy then used as a sealer - and always unse non-shrinking (is it the right translation?) resins, because some products are pre-thinned.

best regards Michael

04-02-2013, 10:55 AM
True, but it can increase penetration without changing the physical characteristics of the epoxy (read the Gougeon article). Yes to what Todd is saying. More specifically, warm the surface to be coated BEFORE applying the epoxy. Then when the resin is applied to the warm surface it will both thin out and be drawn into the pores of the wood as it all cools. If you heat the surface after applying the resin you are likely to see bubbles form in the coating as air in the wood (and solvents if you've added them) out-gasses. This will also happen if you're working outdoors and apply resin to a shaded surface that is then exposed to the sun.

04-02-2013, 03:36 PM
Thanks for everyones inputs. I will not thin my epoxy.

04-02-2013, 07:52 PM
I've used epoxy thinned with alcohol, but I was unsure enough of its sealing properties, I put several coats of paint on top of it. It's entirely possible the several coats of paint would be sufficient, but I think the coat of thinned epoxy gave me a better surface to paint.

From Smith's website:

Clear Penetrating Epoxy Sealer™ (CPES™) consists of a tough, flexible resin system in a solvent blend which dissolves the sap, oil and moisture found in wood. The resin system is derived from natural wood resin and develops a chemical adhesive bond to the wood fibers themselves.

The wood is strengthened while allowing normal expansion and contraction with changes in temperature and humidity.
When repairing wood that has mild dry rot, the CPES will migrate along the abnormal porosity which dry rot fungus creates within the wood, as long as it continues to be fed in. Impregnation of wood with CPES changes the cellulose of wood (which fungi and bacteria find tasty and easily digestible) into epoxy-impregnated cellulose which resists further attack by fungi and bacteria while reinforcing the wood, accomplishing restoration.

Paint or varnish will last longer because the paint has a strongly attached, chemically compatible surface to bond onto.

Now, it strikes me that you don't get the wood expanding and contracting if you have actually sealed it. On the other hand, I'm not sure what, other than encasing the wood in fiberglass and resin, would fully seal it. What most people seem to mean when they refer to 'sealing' the wood is pretty much what CPES does -- bond to the wood and give the paint a better surface to bond to. The result is less checking in plywood, which is after all the goal. But if you really want to seal the wood so it's safe from the intrusion of water, it's going to take a thicker barrier. You can get it smooth on a flat surface by putting it on with a plastic spreader, and just scrape off the excess. I'm not sure that does any better than CPES, but I did this on Black Swan's centerboard (then painted it with several coats of enamel) and it hasn't checked, in spite of several years of banging around. But I suspect I'd have the same result with thinned epoxy and several coats of paint.

Richard Smith
04-04-2013, 05:02 PM
Ok, just to stir up the bees . . .

I've used all of the above techniques and still do - depending on what I'm trying to accomplish.
Additionally, I use Interlux 2333N as a thinner when I want a slow cure and deeper penetration. It smells just like CPES and is just as deadly. Sometimes I heat the wood AND thin the epoxy. Generally, I try to heat the wood regardless of whether I'm using thinned epoxy or not - for the above stated reasons.

Sometimes I use a varnishing type approach. I start with epoxy thinned to about the consistency of water and make subsequent coats with less thinned epoxy, eventually ending with unthinned epoxy.

I've even been able to get epoxy to stick to ABS and PVC plastic as a primer for latex paint. I use PVC/ABS pipe cleaner as the epoxy thinner and then hot coat the latex while the epoxy is tacky. Works great!

MN Dave
04-04-2013, 07:11 PM
Ok, just to stir up the bees . . .

I've even been able to get epoxy to stick to ABS and PVC plastic as a primer for latex paint. I use PVC/ABS pipe cleaner as the epoxy thinner and then hot coat the latex while the epoxy is tacky. Works great!

Well, Oatey cleaner is MEK and acetone, some also contain THF and cyclohaxanone. This is very similar to the solvent mix in CPES. It makes sense that the solvent would allow for some intermingling of the epoxy and PVC, which would enhance the bond strength to the plastic.

I don't know which epoxy you use, but I would not advise hot coating West Systems with a water base paint because the hardener is apparently water soluble. I would expect the results to vary between brands of epoxy and latex, so any advice along those lines should be specific about which resin system and paint combination worked. For that matter, I wouldn't expect you to get good results with any combination of epoxy and latex. If you found one that works, that's good, but don't be surprised if another combination doesn't.

The idea with thinners is to use a solvent that will evaporate before the epoxy sets up so as to avoid making a plastic sponge. Go back to post #10 and follow the links. West found that thinned epoxy lets the wood absorb more moisture than straight epoxy. Over coating the thinned epoxy with straight epoxy reduces the moisture pick up.

Whatever you do, DO NOT use these solvents on Lexan or any other polycarbonate. Ketones are death to PC.

West System International
08-20-2015, 08:35 AM
As others have mentioned above, thinning will change the characteristics of your epoxy so it's worth weighing up the pros and cons before you go ahead with the process. You may be interested in reading this article which details best practice for thinning epoxy: http://www.epoxycraft.com/thinning-epoxy-best-practice-reducing-viscosity-west-system-epoxy/