Thinning Epoxy
 

I've seen references to using Thinned Epoxy as a fuel proofing agent for firewalls. What would be a good thinning agent for 30 minute epoxy without spending a fortune? I have Turpentine, mineral spirits, nail polish remover available, can purchase alcohol from the local drug store. Any recommendations as to which would be better?

Thanks,
Brad

RE: Thinning Epoxy
 

91% isopropanol works pretty good for that. It's also great for getting uncured epoxy off of your hands.

RE: Thinning Epoxy
 

The typical recommendation is denatured alcohol or acetone, both of which are available from places like Home Depot or Lowes. You can also just warm the epoxy, it will be thinner. I don't bother with epoxy, I use Balsarite. I brush 3 or 4 coats on the engine area, fuel tank area if I can reach it, and around any of the seams where the covering meets wood, such as the wing saddle area.

RE: Thinning Epoxy
 

I have learned NOT to use the 91% alcohol from the drug store. The water in it will make the epoxy rubbery when it sets up, if it sets up at all. Sometimes it still feels tacky even after a couple of days.

I use the denatured alcohol as piper_chuck said. Much better results.

RE: Thinning Epoxy
 

Basically, the solvent has to evaporate out before the epoxy cures. The less that escapes before hardening, the more “rubbery” the epoxy will be. So longer cure time epoxy is better to thin than shorter. And faster evaporating solvent like acetone is better. Lacquer thinner works too. Long cure epoxy dries nicely with denatured alcohol added up to around 20% or so. I haven’t tried thinning any more. No reason too. Always try it out a test mix first to see if you end up with an acceptable cure.

Multiflyer

RE: Thinning Epoxy
 

The whole problem with this "what do I use to thin epoxy/alcohol" question is that there are three different types of alcohol commonly available out there and all of them are available with different percentages of alcohol. "Alcohol" can refer to any of a great number of different compounds and simply means that one carbon atom has an "OH" on it.

Methanol="wood alcohol"= "methyl hydrate". This in glow fuel. TOXIC!. [:'(] Available in concentrations up to 99% in paint stores. Also sold as gas line "antifreeze". Also sold as fondue fuel. The other 1% is water and trace contaminants. Produced by the oil industry. Is one carbon long.

Ethanol="grain alcohol"="spirits". This is the stuff in beer/wine etc. Usually taxed like crazy because people like to make things out of it... like rum punch. [sm=bananahead.gif] To allow it to be sold without all the tax, it can be bought as "denatured alcohol". Denatured alcohol is ethanol to which relatively small amounts of things like benzene or methanol (hence "methylated spirits" ) or any combination of about 20 or so other chemicals have been added. These additives make it taste horrible, make it poisonous and make it darn near impossible to distill into anything even close to being drinkable. A real good chem lab might be able to clean it up, but you would spend so much time/effort/money doing it and need all the right equipment that if you have all that money, you may as well go out and buy a bottle of whisky (Single malt please!). Produced by fermentation... yeast and sugars. Is two carbons long.

Isopropanol. Often sold as "rubbing alcohol". ... but not all "rubbing alcohol" is isopropanol. Confusing huh? [sm=confused.gif] Not as toxic as methanol, but still toxic if you decide to make rum punch out of it. Available in concentrations of up to 99%... usually in drug stores. the other 1% is water and trace contaminants. If you're looking for it in adrug store, you might have to ask for it as some jurisdictions have restricted it's sale because the idiots who run illegal drug labs use it as a solvent. Often used in antiseptic wipes prior to giving injections. Again... an oil industry product. Is three carbons long... the "iso" part just refers to which carbon the "OH" is on.

"rubbing alcohol"... a non specific term for either isopropanol or denatured alcohol.


For what it's worth, I use 99% isopropanol to mix with epoxy. Denatured alcohol works too... just get the highest percentage you can and don't make it too thin. The other thing I've found that seems to work OK is finishing resin... really thin watery epoxy with about an eight hour cure time.

Sheesh... and here was me trying to forget all that organic chem! :)

RE: Thinning Epoxy
 

CP140: Thanks for the chemistry lesson. I have a degree in Biochemistry, but I'm sure there are some out there that this helped.

To all: Everyone discussed the "alcohol" thinner, but what about turpentine or mineral spirits? I can't find any performance data on these as thinners, so I expect they are unacceptable. Turpentine is distilled from pine resins, so it probably has very good penetration of the wood, but might affect the wood fibers before it evaporates out. It also has a much lower vapor pressure than isopropanol, so it might not ever fully evaporate. Mineral spirits may even be more problematic, since I believe they can be used to dissolve and loosen epoxy (used as a mold release in some epoxy casting methods).

Any actual experience with either of these? Perhaps its time to conduct some experiments in "the lab," otherwise known as my basement workshop!

Thanks,
Brad

RE: Thinning Epoxy
 

Methanol from your local paint store works well. Better than acetone in my experience.
Methanol is called "methyl hydrate" at the paint store.

RE: Thinning Epoxy
 

Go for it. If I actually need to thin epoxy I use one of the two I mentioned. They're easily available and I keep a can of each in the shop for other uses. I've found serveral studies online showing the results of thinning epoxy. The conclusions were that thinning weakens the epoxy, but for fuel proofing, that's not a problem.

RE: Thinning Epoxy
 

Hi!
Why use 30min epoxy??? I use thin Ca glue for this...if I ever bother to seal the tank area at all... Most of the time I don't bother.
Finishing resin epoxy is good though much better than ordinary glue epoxy that has to be warmed with a airgun or thinned with a little methanol (not too much and the epoxy gets rubbery). Better is 2 part car or boat acrylic or polyuethane lacquer (Sikkens or PPG).

Regards!
Jan K
Sweden

RE: Thinning Epoxy
 

This is my first build. I had to purchase 30 minute epoxy and 5 minute epoxy for some parts. Consequently, I have way more 30 minute epoxy than I'll use in the next 12 months. I figure if I can thin it with turpentine or mineral spirits, which I already have, I can use up what I have and not worry about shelf life.

Brad

RE: Thinning Epoxy
 

I JUST did exactly what you are talking about on a new composite pattern airframe I just completed. I used 20 minute epoxy cut 50/50 with denatured alcohol just like Chuck mentioned. 30 minute epoxy should work fine as well. I tell you this because you dont jack around with an expensive airframe like this, you use what works. What works is denatured alcohol OR acetone. I mixed up about 2 tablespoons of epoxy and 2 table spoons of denatured alcohol and used a paint brush to put a very thin watery layer of the stuff all over the inside of the chin cowl, engine compartment and pipe tunnel. Its totally fuelproof and ads very little to weight to the plane. It consequently adds a nice clean shine to everything it covers as well.

RE: Thinning Epoxy
 

In case you are not already aware of this, high temperatures really accelerate the curing of epoxy resin. You can glue something with the slowest curing epoxy made and put it in a warm oven and it will be hard in about 10 or 15 minutes. A car parked in the sun with the windows rolled up makes a good oven for this.

RE: Thinning Epoxy
 

Only thing is Im not so sure if accelerating the cure speed doesnt make the epoxy brittle. I sometimes use micro balloons in my epoxy and I get the same effect if I add too much. I know that it gets as hard as glass and it is real brittle if you just set some out and see what happens. If it cures normally it is hard but not nearly so brittle, its a little more pliable which makes me feel that it has more give while maintaining its strength. Just my opinion, not scientific fact.

RE: Thinning Epoxy
 

This is quite interesting. I always thought isopropanol, denatured alcohol, and 'rubbing' alcohol were all the same thing, only that rubbing alcohol had some water in it. Thats why I steer away from it. Thanks CP140 for the lesson.

I don't use CA on firewalls because I think it will eventually break down with constant exposure to fuel. Correct me if I'm wrong on this- it's what I was told.

RE: Thinning Epoxy
 

Hi!
No problems using thin Ca on firewalls.

Jan K

RE: Thinning Epoxy
 

Just to close out this discussion, I ended up purchasing some PVC Pipe Cleaner at Home Depot. A small can (8 oz) was only about $4. It consists of a mix of Methy Ethyl Ketone and Acetone. I needed it to work on the ABS plastic cowl, but didn't want a full quart. Works great to thin the 30 minute epoxy. Allowing to cure overnight gives a very nice hard finish.

Brad

RE: Thinning Epoxy
 

If you have MEK, that is the best epoxy thinner I've tried. Second on my list is acetone, and third is methanol if I'm in a pinch. I'd avoid any product that has water in it, such as rubbing alcohol. I know you can drive the water out, but why bother. Just get some MEK or acetone. Sometimes you can get MEK at hardware stores (my local Menards carries it with the paint thinners), if not try a auto paint store.

As for heat cure, depending on how it is done and depending upon the epoxy, it typically strengthens the epoxy. The heat encourages more bonds to form, resulting in a stronger structure. The heat MUST be applied during the cure and cannot be applied after the fact. A quick shot with a heat gun won't do it. You generally have to "bake" the epoxy at about 140F-150F for hours (I typically bake for 12+ hours) as it cures. But this doesn't really apply to epoxy as a fuel proofer and is more along the lines of making composite parts. I offer the tidbit only because the question of heat was brought up in a previous post. Cheers.

RE: Thinning Epoxy
 

I use epoxy finishing resin as it is already thin and doesn't leave that waxy surface.