Since this topic has come up once in a while, I thought I’d share this article I found on the internet
Hi:
When glassing an airplane wing with 5 oz. glas ussually i use some methanol (alcohol) to thin the epoxy. It all depends on the purpouse of thinning, if the piece to mold is structurally critical, i would have more care.
Tato Lazo
Hi Tato
I’ve been using acetone to thin WEST epoxy and have been using it as a sealer for the balsa hull of the RG-65’s I’ve been building. Being very thin, it absorbs quickly, doesn’t weigh much, and the acetone “flashes off” very quickly and allows a fast cure.
Have used a double coat for inside and outside of hull and seems to seal wood very well from water. Also able to paint over it with regular spray paints - nothing exotic required.
I’ve never been all that fond of using acetone to thin epoxy. Seems as though as soon as you stop stirring, the mixture starts to separate. I’ve had much better luck thinning epoxy with good ol’ styrene. It stays mixed. Usually I will only use thinned epoxy as a pre-treatment for wood. The first coat is thinned with upwards of 25% styrene & applied to the wood. It goes on like water & definitely gets into every nook & cranny. The second coat is thinned with less styrene – say 10%. The subsequent coats are all full strength epoxy. The styrene in the thinned mixes unfortunately causes porosity as the microscopic bubbles of solvent flash off & burst leaving microscopic craters on the surface. To combat that, each coat is allowed to dry at least 24 hours, then sanded & cleaned before the next coat of epoxy is applied. Even though thinning degrades the physical properties of the epoxy, the thinned epoxy is better able to penetrate into small crevices. Full strength epoxy mixes just might not be able to ooze into every spot you’d like. I’d rather have somewhat degraded epoxy in there, rather than no epoxy at all. The best part of using styrene is the smell. It smells like boats. *** Styrene is REAL flammable (gasoline is girlie girl compared to it) – be extra careful ***
Happy Yachting - Kip
If I’m in a situation where penetration or wetting is a concern, I use Aerospace Composites epoxy, as is, with complete success so far.
Cheers,
Earl
I use an insulated, heated box . It has a light bulb hooked up to a thermostat and it keeps the epoxy at around 100 f. I use west systems and at 100f and it is as thin as I need it. It also helps to have whatever you are working on be warmed up too.
Ref. use of styrene.
Yes, you must be ‘some guy’. Availability of this aromatic hydrocarbon must surely be very restricted. When I was at sea in the seventies, we used to carry bulk styrene monomers, a few thousand tonnes at a time, and it was a very nasty cargo. It was loaded in the Mississippi and had to be discharged in Europe in less than fourteen days, as the polymerisation inhibiter which was mixed in just prior to loading would loose effect in just over two weeks. If the cargo temperature started to rise then we had orders to pump the total cargo overboard and forget about the local environment, as the temperature rise would accelerate until there would be a fireball. Goodbye ship and forty crew! Boom! We all had our own personal air breathing sets.
I wouldn’t even think of handling it in a domestic situation.
Cheers
Ralph
So - ummm, Ralph … Plutonium is a definite no-no as well? 
The reason I went to epoxy is that I can use in lower level or in garage and not worry about my wife hollering …“What’s that awful smell?” Of course, at the time I thought she was referring to me ! 
Epoxy has little, if any smell, and while acetone used as thinner will have a smell, but one can also use alcohol to thin… (but not the 30 year old Scotch in the liquor cabinet). No, or very little smell has allowed me to move my building indoors when weather gets cold as was one of the major reasons I moved away from polyester resin and styrene. I don’t even like using styrene to thin polyester resin.
Ralph,
It must not be that restricted as it’s an over the counter buy at the fiberglass store. $4.55 per pint. There isn’t anything on the can about blowing up 40-man ships but then again that probably wouldn’t be a very good marketing ploy.
Happy Yachting - Kip
Glasplies of Southport, Merseyside: £16 for 15 lites, or £8 for the handy 2.5 litre size.
It’s still very nasty stuff. :zbeer:
Epoxies used for resin infusion are specifically made thinner, for application requiring just that.
I like the method above using heat. Good idea :graduate:
If you ‘Google’ - ‘Styrene Monomer’ -and click on the link to:- www.styrenemonomer.org/2.1.html then there is good info on the product. Then, if you drop down to the next entry:- Lyondell Styrene Mononers, to Technical Information, to ‘Vessel Specification’ you will see that what I said before is basically true . It refers to 45 days with temperatures around 85F. On a ship, where lives are at stake, this time and temperature is reduced conciderably, the time by a factor of three. Having loaded in the Gulf of Mexico with elevated sea temperatures, we were all glad to get clear of the inshore Gulf stream nd into cool waters. Unless there is now a new type of self polymerization inhibiter in the small quantities < 2ltr that you are talking about, compared to 1000x1000 ltrs in each tank that I was involved with, then still be carefull and do not allow the product to be exposed to raised temperature. I’d be tempted to keep it in a refrigerator.
Happy days!
Ralph