Cowl epoxy thinner?

I'm about to start working on my engine cowling again. I can't remember what I used to thin the epoxy I used on the cowling. I think it was acetone, is that right?

bhester said:

I'm about to start working on my engine cowling again. I can't remember what I used to thin the epoxy I used on the cowling. I think it was acetone, is that right?

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Should not thin epoxy. Composite folks will tell you that. Metal people will differ. You choose......

Per Gougeon Bros. Wet System: (http://www.epoxyworks.com/14/ThinningEpoxy.html)

•Adding a volatile solvent extends the pot life and cure time of epoxy and jeopardizes the reliability and predictability of cure. Additionally, with slow rate of cure, it takes longer before work can be sanded.
•Adding volatile solvent may cause shrinkage of the cured epoxy. Applying thinned epoxy in large, confined areas (like consolidating a large pocket of rotted wood) is likely to trap some of the solvent. In thick applications, the epoxy cures very quickly and not all of the solvent has time to evaporate before the epoxy hardens. Over time, the solvent works its way out and as this happens, the cured epoxy shrinks and in many instances cracks. Shrinkage also causes print through. You may have a surface sanded smooth only to have the resin shrink. This shrinkage often reveals the texture of the substrate. Shrinkage can continue to be a problem until all the trapped solvent works its way out of the cured epoxy.
•Adding solvents, especially acetone, alters the color of the cured epoxy. While the effects are not immediate, adding acetone to epoxy causes the color to change from slightly amber to very dark amber.
•Adding solvent results in a temporary reduction in viscosity. Volatile solvents evaporate quickly as they are agitated during brushing or rolling, causing the viscosity to continually change as time passes.
•Adding solvent to epoxy may damage the substrate. Many materials (Styrofoam™ for example) are not attacked by epoxy but may be attacked by the solvent used to thin the epoxy. Be certain to test the substrate with the solvent before using it to thin the epoxy.
•Adding volatile solvent to WEST SYSTEM epoxy has some adverse health and safety effects. WEST SYSTEM epoxy components are nonflammable but the chance of fire or explosion goes up in proportion to the amount of solvent you add. Also, the vapors of many volatile solvents are hazardous to your health and proper ventilation is mandatory to prevent inhaling harmful quantities of them.
•Adding volatile solvent to epoxy which is then applied as a coating may cause problems with various regulatory agencies. If your business is inspected for air quality, adding volatile solvents to WEST SYSTEM epoxy may make your business non-compliant.
•Adding solvent to epoxy to enhance fiberglass wet-out will result in more "drain out" of the resin on a vertical surface. The fabric will wet-out quickly but it may become resin starved when too much epoxy runs out of the fabric.
Does thinning epoxy make sense? In some situations, thinning is appropriate. In others, it is not. We feel that in most circumstances using heat to thin epoxy is preferred to using solvents. As long as the epoxy does not overheat during cure, the full physical characteristics of the cured epoxy remain. Adding solvent is a quick, simple method of thinning epoxy, but the strength and moisture resistance of the cured epoxy are significantly reduced.

System 3 Clearcoat

Best epoxy for pinholes IMHO. Developed for below waterline on wooden boats. Flows like water.

Mike

Yes - Acetone

There was an article on the sealing process to use if you are going to fly before painting written by Ken Scott in the RVator (I found after I volunteered to give a copy to someone else that I don't have them anymore). He said basically mix the epoxy as normal then add in an equal volume of acetone. stir well and paint it on with a brush - I think two coats. I followed his procedure and it worked very well. Then the plane was painted and that worked very well also. The plane has been painted for almost 7 years now and there have been no problems. I used micro balloon home brew filler slathered on and sanded off before the thined epoxy sealer.

Bob Axsom

flyingriki said:

Should not thin epoxy. Composite folks will tell you that. Metal people will differ. You choose......

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I'm a composites folk, and when it comes to filling pinholes I will use thinned epoxy.

Thanks, Bob K.

Look.....do a simple experiment. Mix two batches of epoxy and hardener in two separate cups. Add 50% solvent of your choice to one of the cups. Go away, come back tomorrow. Now do some comparison tests....a scratch test to check surface hardness, peel the cups and hit the lumps with a hammer, whatever works for you.

Dan;
I wish you had been more direct.
I'm nobody, but I do have one data point.
I used unthinned West epoxy on the inside of my cowl. I used a paint brush, and spent a little extra time and care to work it in and work out bubbles etc.
I thought it turned out great.

I agree with Dan! Test your epoxy and thinner first. My experience with west systems as per vans instructions ended up like chewing gum. Very bad...

Unthinned on my cowl worked great just like Scott suggested.

Doug Gray

DanH said:

Look.....do a simple experiment. Mix two batches of epoxy and hardener in two separate cups. Add 50% solvent of your choice to one of the cups. Go away, come back tomorrow. Now do some comparison tests....a scratch test to check surface hardness, peel the cups and hit the lumps with a hammer, whatever works for you.

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Of course, if you use 50% solvent, you are using far too much. About 3% acetone for West Systems stuff is all you need to have a significant impact on viscosity - maybe even less depending on your goals.

Rather than listen to airplane builders of any stripe, why not go to the epoxy manufacturer? West Systems has a nice description of what happens when you thin their epoxy at http://www.westsystem.com/ss/thinning-west-system-epoxy/ .

Based on West's writeup, I slightly thinned my epoxy (about 3% acetone).

Alcohol or acetone is just fine. Not terribly aggressive and evaporates quickly. Strength is not at issue, just filling the holes so oil doesn't leak in. I'm also not looking for baby-butt smooth. When I have a bit of a West Systems batch left over from a lay-up, I thin with however much it takes, proportions are non-critical, and brush, pour/brush, whatever (great word!), on the inside of the cowls (especially lower) surfaces. At least two coats total. Use your judgment. If you're spreading it real thin, add coats, fairly viscous you can get away with two.

John Siebold

I too thinned mine on both cowls. To apply it I used a three inch high nap roller and applied three coats (with three different rollers).

Note, it will take a lot longer to harden than straight epoxy and after a day or two, what's lever over in your cup may be rubbery. However, it will harden. Oh, and I only waited a day between each coating.

Then I applied some heat shielding to inside bottom of the cowl. Use a wallpaper seam roller to press it on and you will get a good finish.

That looks GREAT!

Bill, from the looks of your lined cowl we used the same high cost but really effective thermal shield/insulation. Mine is OK but not to the level of perfection that I see in yours. I found that I had to use 3M weather strip adhesive to reattach it in some areas of compound curveswhere I made radial stress relief cuts during the original application. It looks like that wall paper tool may have helped avoid that little complication.

Bob Axsom

If your having trouble getting your epoxy to flow its because its too cold. Add a little heat with a hair dryer in one hand and your brush or squeegee in the other. Built 2 Cozy MKIV's at my house and we went through 7 or 8 hairdryers doing so. Never had a reason to thin epoxy with a "thinner" and would never do so. A little heat can be your best friend when working with composites.

Lacquer Thinner

Used a little Lacquer Thinner per West System instructions wiped it on with a brush and then used a small foam roller seemed to work pretty well. I'll sand it in a couple days and add another coat hope I'll be ready to prime it after that. I've only been flying it for 5 yrs since I did this the last time. Got to get serious about painting. Just hate to work on it when it flys so good.

Bob, here is what I did on my cowl. I used the same material as a heat shield and then brushed on a coat of epoxy around the edges of the heat shield. It sealed it up and so far nothing is coming loose. Not sure about the long term yet though. If you ever get bored fly over to Harrison and I'll buy lunch.
741-eleven fifty five.

I thinned epoxy 50% with acetone, painted it on, sanded most of it off. Gave it a skim coat of SuperFil, sanded it off, rolled on three thin coats of SuperPrime, sanded it off. Rolled on three more coats, sanded is to a smooth finish, added a coat for UV protection, sanded to perfection.

I just did some pinhole filling last week. Take your favorite epoxy and measure it out in your mixing cup. Nuke it in the microwave for 10 seconds, then mix it up. Water-thin. Squeegee it on and sand off.

just mix up some West system 404 with your epoxy fiberglass resin and hardener, wipe on enough to fill the holes, but not mounded up on the piece, let it dry, then block sand it. Its pretty dang simple..

Thinning the resin, fancy materials and fillers and/or elaborate techniques are all a waste of time.. fill it with what its made of, block sand it correctly and SHWAAA BAMM!! an pin hole free part, that is also free of molding imperfections!