Denatured vs 91% Isopropyl Alcohol - RC Groups
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Jul 04, 2008, 10:29 AM
Cobalt Corsair

Denatured vs 91% Isopropyl Alcohol

I've read various suggestions about thinning epoxy and they come down to either denatured or 91% isopropyl alcohol. I've used both with good success and don't seem to notice a difference.

I can get 91% isopropyl at Wal-Mart very cheaply but it's been so long since I bought denatured that I don't know what it costs.

Opinions about effectiveness of either one as an epoxy thinner?

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Jul 04, 2008, 01:31 PM
Will fly for food
You shouldn't use a thinner with epoxy. It degrades the bond and the structure.

Denatured alcohol is ethanol (C2H4OH) with methanol (CH2OH) added to make it non-drinkable.

Isopropanol (C3H7OH) is a different acholo.

Both work equally badly in epoxy.
Jul 04, 2008, 02:28 PM
Cobalt Corsair

If the sailplane guys do it...

If the sailplane guys thin their epoxy then I guess I'll continue to thin mine (although epoxy finishing resin is my choice for coating, rather than bonding).

Since those guys are the r/c fliers' high priests of dweebiness for weight and materials I guess I'll take their lead.
Jul 04, 2008, 11:58 PM
Bags for hire.
Chad Sullivan's Avatar
Thinning Epoxy is OK to do for appropriate applications. It does degrade some of the properties of Epoxy. So it is not appropriate when your application is going to max out those properties.
If you want to apply a skin to a molded bead foam airframe so you can apply a nice coat of paint, thinning Epoxy with denatured alcohol is a good way to get the Epoxy thin enough to wick out into the cloth. You will use less Epoxy this way keeping any weight gain to a minimum. Because the bead foam airframe was probably designed to be structurally sound in bare foam there is no need to worry about the negative effects the Denatured alcohol has on the Epoxy.
Beyond an application like this though it is better to use a full system Epoxy like West systems. The Part A resin is low viscosity, and the Part B (Hardener) can be chosen for it's viscosity, and cure time. Usually a lower viscosity hardener goes with longer cure times, but you gain work time.
You can heat/warm the parts separately to make them even lower in viscosity, almost to the consistency of milk. If you want to use a fast (6-8 hour) hardener but you feel like it's still a bit thick.
You definitely don't thin Epoxy with alcohol to bag wings.

Oh, and if you do use Denatured Alcohol , use protective gear. Vinyl gloves and a respirator. You body metabolizes this stuff into Formaldehyde.
Jul 05, 2008, 05:35 AM
Registered User
vintage1's Avatar
Exactly. When glass skinning, ou re not really exepecing or needing a massively strong bond: allyou are ding is creating a very strong galss skin: and the epoxy simply is there to part fill the weave and stop it sliding over the surface. And if over foam, make a surface that paints won't get through to attack the foam.

Arguably the water based acryclics are just as good, or better.

Save epoxy for making very strong gap filling glass reinforced fillets...
Jul 05, 2008, 09:59 AM
Will fly for food
I use heat to thin my epoxy, works wells, doesn't degrade the properties. And gives a shorter cure time for the given hardener chosen.
Jul 05, 2008, 10:06 AM
Cobalt Corsair
Thanks for the replies.

I have an email request in for a response from one of our club members. His company specializes in industrial epoxy and acrylic applications, and he's a builder of giant-scale war birds. I'll get his take and report back.
Jul 05, 2008, 10:35 PM
Will fly for food
From WEST Systems:

Thinning Epoxy

There are epoxy-based products specifically designed to penetrate and reinforce rotted wood. These products, basically an epoxy thinned with solvents, do a good job of penetrating wood. But the solvents compromise the strength and moisture barrier properties of the epoxy. WEST SYSTEM epoxy can be thinned with solvents for greater penetration, but not without the same compromises in strength and moisture resistance. Acetone, toluene or MEK have been used to thin WEST SYSTEM epoxy and duplicate these penetrating epoxies with about the same effectiveness. If you chose to thin the epoxy, keep in mind that the strength and moisture protection of the epoxy are lost in proportion to the amount of solvent added.

There is a better solution to get good penetration without losing strength or moisture resistance. We recommend moderate heating of the repair area and the epoxy with a heat gun or heat lamp. The epoxy will have a lower viscosity and penetrate more deeply when it is warmed and contacts the warmed wood cavities and pores. Although the working life of the epoxy will be considerable shortened, slower hardeners (206, 207, 209) will have a longer working life and should penetrate more than 205 Hardener before they begin to gel. When the epoxy cures it will retain all of its strength and effectiveness as a moisture barrier, which we feel more than offsets any advantages gained by adding solvents to the epoxy.
Jul 06, 2008, 09:53 AM
Registered User
bullseye000's Avatar
I only thin epoxy when I am fuel proofing parts on glow models. I prefer acetone, it's seems like less is needed to get the desired consistency. Brushes on like paint and leaves a nice even finish. Haven't had any problems yet. I would never thin epoxy that was going to be used in an application that relied on the strength of the bond, like a firewall or wing joint. For finishing resin if it's too thick then you're either using the wrong product or it's old an going bad.
Jul 06, 2008, 10:06 AM
.: Looking for Thermals :.
Ricardo RW's Avatar
For fiberglassing small patches I thin my epoxy with acetone, with better results than with isopropilic alcohol.
Jul 06, 2008, 01:01 PM
Space Coast USA
hoppy's Avatar
91% IPA contains 9% Water - 99% is also available at a higher cost. I found it at a local drugstore

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