I've got some GreatPlanes Pro epoxy finishing resin that I bought to use for fiberglassing. This is pretty thick stuff and will absolutely have to be diluted to use with the delicate fiberglass cloth. I know people use alcohol to dilute epoxy but I'm afraid that I'm not up on my types of alcohol.

I picked up something that in Japanese is labeled as "sterilizing ethanol." There was also something called "Flammable ethanol" (which was much cheaper). What kind of alcohol should I be using?

BTW, three hours later my "text mixture" of the epoxy is still soft and sticky.

You want denatured alcohol from the hardware store. Not medical alcohol, it has too much water in it. It only takes a touch to thin epoxy, its easy to mix too much and end up with the gooey mess you have now.
Edwin

Hmmm...so what IS denatured alcohol? Does it have a chemical name I might ask for here in Japan? As far as the "sticky" epoxy I have now...well, I haven't diluted it all all yet, just mixed (small) equal amounts.

for what it is worth,I was always told to delute no more than 10%.

According to Wikipedia denatured alcohol is also used as a fuel. So maybe the "burnable ethanol" is what I need.

In terms of the current usability of the epoxy resin, it just tears the glass apart when I try to brush it on. That is, I brush on a coat of epoxy, then lay on the glass, but when I try to "paint" on another coat to saturate the glass, it just tears the glass apart.

Good think I'm trying this on a test piece!

Five hours later. It's now to a "cured but sticky" stage. I'll give it 24 hours and see how it seems.

Acetone is the best thinner for epoxy. Alcohol can leave the finish rubbery for days and days. The only time I use alcohol is when I am glassing foam.
Actually I used to use alcohol, then I read some stuff that changed my mind. On a chemical reaction level, the acetone supposedly does a better job. All I know is that when I thinned with acetone, I got a quicker, harder surface.

I'd need to dilute it a LOT more than this in order to use it the way I've seen glass being applied.

Make sure the epoxy you are using is labeled "finishing resin". Standard epoxy either won 't set in thin layers to a hard coat or sometimes won't set at all, seeming sticky. I have used denatured alcohol and laquer thinner to thin epoxy, both have worked for me. Also, you want to put the cloth on dry and either brush or squeegee the resin through the cloth, then using a piece of balsa or wood, scrape off as much resin as you can.

It's definitely finishing resin (from GreatPlanes). So you DON'T apply a later of resin FIRST? I should say here that I have never (successfully) glassed anything.

Also, what are you making. if you are fiberglassing over something, finishing resin is great. if you are making a part, it will come out rubbery. A "real" resin like West Systems should be used for that. as to the "thick", resin, steve hit it on the head, lay down the cloth and pour, spatula on the resin and squeegee it off. A playing card will work real well. If you are hyper concerned about weight. roll some tissue across to absorb excess resin. I have never needed to cut down finishing resin. Just 5 minute epoxy.

DJ

I've been using acetone for decades. As the acetone evaporates the epoxy will get thicker until it cures hard. It works very well for thin layers for fuel proofing the firewall and fuel compartment.
When applying glass to a fuselage or any other part, wet (saturate) the cloth with resin on a piece of clean cardboard, then apply the whetted cloth to the part. You can then use a squeegee, roller or stiff bristle brush using a stippling motion to remove the air bubbles, smooth the cloth out and remove excess resin.

"Denatured" alcohol is 90% ethanol (grain alcohol) + 10% methanol (poisonous)

I to am a fan of thinning epoxy with acetone and not alcohol. I thin the finishing resin 25-30% for the first coat. I lay the glass cloth on the airframe and lightly brush with a drafting brush. That creates a little static electricity to hold the cloth down and smooths out any wrinkles. Then I pour a small amount of thinned resin on the cloth and spread it with a playing card. Light pressure on the card as it will pull the weave of the glass otherwise. The epoxy thinned at 20-30% is almost the consistency of water, so a little goes a long way. I squeege this first layer to a flat sheen. I let that cure, usually just over night in a 65 degree shop and hit it lightly with 100 grit paper. Then I mix up a second batch of epoxy and thin it 15-20%. Again pour on and spread with a playing card. Squeege with the card until the sheen is semi-gloss and let it cure. You will have a thin, strong finishing surface to work from. Lightly sand with 120 grit and you are ready to move onto primers and fillers.

Thanks for all the replies so far. I'll be using the finishing resin with light(est) weight glass for several purposes. My most immediate need is as reinforcement of a cracked wing strut (see my Snipe thread). I've glued the strut parts together, added some wire "staples" and would like to finish it off by wrapping it with glass.

Here's what I've tried so far:

1) unthinned resin (equal parts) first applied to the struts and then wrapping the glass, then put on second layer of resin. This doesn't work well as spreading the resin starts to tear apart the VERY DELICATE glass cloth. I can't imagine how I'd "scrap it with a card" for this use. Even the undiluted resin is taking forever to cure. 12 hours later and it's still slightly tacky and "greasy."

2) As an experiment last night, I diluted the resin with nail polish remover (acetone) until it was a sort of "thick cream" consistency. 8 hours later, it's still "squishy."

So far, I'm not a happy camper.


I've both read articles (in FSM) and watched youtube videos (for example the B1 Bomber guy) where the glass is laid down and essentially "painted on" with what must be very thin resin. This looks like the best technique to me.

I read (again in Wikipedia) that the only reason ethanol is "denatured" for industrial uses is that otherwise there'd be no way to regulate the sale of cheap drinking alcohol! So they add the poisonous and unpalatable ingredient specifically to make this "cheap alcohol" undrinkable!

Use acetone Abu. The term 'denatured alcohol' covers about 50 different 'grades such as 3A(ethanol diluted with 10% methanol), 2B (ethanol diluted with 5% benzene) etc. You could probably find a list on the web but acetone works just great.

What sort of cure times should I be expecting? I've been extremely frustrated over the years with any of the "slow cure" epoxies I've worked with, so I use 5-minute epoxy exclusively in my building. The "30-min" stuff I can buy locally takes hours to set and the "90-min" stuff takes forever (if at all).

And that's why I special ordered the Pro epoxy finish resin from GreatPlanes. But it's also taking a LONG time to cure fully. On the bottles it says it cures in 3 hours and has full strength in 24. But 3 hours later it was still VERY sticky and 12 hours later it's still tacky and greasy (and certainly still giving off strong odors). It's definitely still "rubbery."

I'm tempted just to go back to using my tried and true 5-minute epoxy.

I use MEK at a mix of 1/3 A, 1/3 B, 1/3 MEK. Works great and does harden, but does take longer to cure. For fiber glassing a plane, I use ZAP Finishing Epoxy. Use a brush to brush the wood first, then lay the glass out and try to dry brush it until all glass is wet. You can apply more resin, but I try to keep it to a minimum.

Ive been doing fiberglass for years. I buid molds and manufacture parts out of them. All using epoxy. I agree with the acetone solution. Ive been using that for years to thin epoxy resin.

From what you are saying, you have tried all that is mentioned and you still are getting a sticky result. I would suspect a bad batch of epoxy. I have had it happen to me before.

A few more things that will mess with you when doing finishing resin. If the humidity is high, like 80% or more, moisture from the air will be attracted to the resin and you end up with what I call a "rubermaid" finish. It will have a whitish hue to it and feel like rubber instead of hard epoxy. It will also have a "sticky" feeling to it.

Check those two things out. Bad epoxy or high humidity during the process.

If you suspect humidity, you can use a large box ( big enough to put your part in your glassing ) and using a heat sorce ( light bulb works good ) put the part in after applying the resin and heat it to about 120 to 140 degrees for an hour or two. It wont take long to set the epoxy off at those temps. The heat will also help with moisture control. Short of a climate controled heat booth, thats bout all I can suggest.

Good luck and hope this helps.

Warbirdguy

Almost 24 hours later, the 1:1 epoxy resin finally seems to have hardened to a non-sticky finish. It's actually great weather now (not hot, not humid) but it might help to set up the heat lamp in the area I work in.

The small sample that was diluted with acetone (aka nail polish remover) at something like 50% is still gooey 10 hours later. This is the same sort of thing I've seen in the past with local 30-min and 90-min epoxies as well as some Z-poxy which I've sort of given up on.

I decided to give the Z-poxy another shot. I mixed that up (equal parts) and was surprised to find that it was MUCH more fluid than the GreatPlanes stuff. MUCH MUCH! In fact, I could brush on the Z-poxy without dilution. The GreatPlanes epoxy resin is impossible to brush on as is (thus my question about diluting it). I've also set up two experiments: 1) chopstick #1 dipped in the Z-poxy and left to cure in my normal workshop location and 2) chopstick #2 dipped the same Z-poxy and left to cure about 1 foot from a heat lamp.

It's now 4:30pm. I'll check them both this evening.

An hour later and the one in front of the heat lamp is already hard. The one in the workshop is thumbprint sticky. I both cases, I've very surprised. Looks like I might use the Z-poxy (which is at least three years old) and put the GP epoxy away for some other use. Maybe it just needs to "age."

If you are going to use acetone, use 100% acetone not fingernail polish remover (that's been diluted significantly with water).

If I can find it. The fingernail polish remover worked well though for cleaning the brush I used for the epoxy.