I have been using the 'Bondo' polyester resin over fiberglass but don't know what to thin it with. If you use alcohol-what kind, isopropyl rubbing alcohol, denatured, or what? And, what proportion? thanks, Bill

acetone but it evaporates quickly ,styrene monomer is better but hard to find and expensive,alchol works with epoxies but not polyesterway back when I was building ducted fans we used k&b OR hobbypoxy thinner 50/50.one trick stafford used and recommended on his b-24 was to apply the resin full strength then use a roll of toilet paper and roll it over the resin to pick up the excess,just keep peeling off the saturated tissue till nothing is picked up.

If you use alcohol get the 91% isopropyl, solvent or stove fuel alcohol. Rubbing alcohol is 50% water. I thin epoxy with acetone. Not sure what a polyester resin would thin best with of those two choices. I know they use styrene for a thinner as one option. Test either acetone or alcohol well before relying on it.

Mix it "pure" first and let it set for a bit and then add the alcohol or acetone.

If you choose alcohol, don't use any that has a percentage on it at all. Isopropyl with a percentage on the label has water in it. Any water is too much. Rubbing alcohols are whatever percentage of alcohol that is listed on the bottle. The rest is water. If it's 70% isopropyl, or rubbing alcohol, it's 30% water.

You can buy denatured alcohol at hardwares and home improvements and it's not very expensive. It's 100% alcohol.

Bondo is a thixotropic. They are often blended with a large filler content. The filler won't thin.

Let us know how the experiment works out.

Lacquer thinner will work also. a little slower then acetone.

O.k., I'll give those a try. I have the denatured alcohol, and may have some acetone left-let you know which works!

G'day Mate,
Acetone is used to clean up polyseter resin & thin it as well, but it is really thin to begin with.
Methanol is used to clean up & thin Epoxy resin, they won't work the other way around.

The cans of Acetone I've bought all have the same information on them. That acetone is an epoxy thinner. And it is, and works excellently well to thin epoxy. So one works the other way around for sure.

I agree with Da Rock,
I looked on my can of acetone lastnight after reading this thread. It says on the can that it thins polyester and epoxy. I have been useing acetone exclusively to thin both and have had great results.

Alcohol is hydroscopic and once open 100% soon becomes 95%. It sucks water out of the air. Iso 91% is about the highest stable concentration that is readily available in drugstores. 100% alcohol would have to come from a lab supply store and would cost plenty. Solvent and stove alcohol are about 91% also. Denatured solvent alcohol still has 3% water when sealed at the factory.

Before you thin Epoxy please read this

http://www.westsystem.com/ewmag/14/ThinningEpoxy.html

Thinning Epoxy with 5% Acetone or Lacquer Thinner can reduce the viscosity by 60% but reduce the strength by 35%.. Alcohol can reduce the strength by 50%
Any one of the 3 can cause shrinking or cracking if to much is added.

Recommended method to thin epoxy is to heat it to around 100 degrees. That will reduce the viscosity 30% and not effect the strength at all.
it may accelerate the cure time slightly.

Next question would be why thin? Only time I do is to waterproof or fuelproof wood. Zero strength required.

If I'm surface glassing I use an epoxy finishing resin that comes nice and thin already.

Interestingly detailed information there. How soon is soon, and when does it become 90%?

If you'd like to get a feel for how fast it sucks water from the air, weigh a can and open it. After however long you wish, close it and weigh it. There should be a rate of evaporation printed right with the rate of water absorption documentation and you could apply both to see just how fast your can chugged down the water out of the air.

Or you could simply use the least watered down alcohol you can find to start off with.

Have been using MEK for the thinning purposes for couple decades now. Is a bit hotter than Acetone, and the graded of raw thinners. Can't just bloop it in, but have to proportion and blend it in otherwise it will develop too soft of spots here and there.

Wm.

All of the polyester resins I have ever used are quite thin enough already. It seems like thinning it any more is totally unnecessary with some risk that strength of the material may be compromised without you knowing it. So why do it in the first place?

Depends on your relative humidity. ;-) My basement dehumidifier sucks about a gallon a day out of the cellar air and we have humidity to spare here in Upstate NY. But you are correct in starting with as high a purity as possible. Then again, I'm still not sure you can thin polyester resin with alcohol. Personally, I use acetone when I thin epoxy, but alcohol and lacquer thinner work also.

Here is a good discussion on thinning from the West Systems site.

http://www.westsystem.com/ewmag/14/ThinningEpoxy.html

And, if you don't believe alcohol sucks up water then let your glow fuel (80 to 82% alcohol) sit around for a year or so in a partially used jug.

Actually, I know for sure that alcohol is hygroscopic. And that's why I only open the cans when I pour some out, and put the lid back on. And when I plan to leave jugs open for a year or so, I expect there to be some water in whatever was in the jug.

One of the real problems our hobby has is that lots of people hear a true statement like "alcohol is hygroscopic" and next thing you hear is that it'll suck the humidity off the walls in the house next door. It's really hard to keep a perspective, but that's what's needed.

Gotcha. We communicate in little blurbs that leave very much out and open to interpretation. Sometimes we think we have explained something well but an outsider who only gets the external dialog has no idea what we were trying to convey. It is astounding how much the human mind can fill in; and mostly correctly. Something that seperates us from computers.

And my favorite alcohol is either Jack Daniels straight or Jim Beam with about 50% water from my little Britta pitcher in the refrigerator. ;-) If you're ever in the area stop in and we can compare notes.