Fairing compound densities

Does anyone have mixed density data for fairing compounds?

I am mostly trying to understand what fairing compounds contribute best to buoyancy and which ones less.

I can mix some up and do my own testing, but not trying to recreate the wheel.

I am using cabot cabosil and sil32 spheres, but any discussion is good for the forum.

 

It will be very hard to get a buoyant fairing compound with Cab-o-sil because it is not buoyant and has a S.G. of over 2. This means that the matrix S.G. must be less than 1, perhaps significantly less depending on how much Cab-o-sil you add.

Microspheres are the only way to go if you definitely need a buoyant filler. Sil-32 microspheres have a S.G. of ~0.2. Assuming full compaction only 33% of the volume will be matrix. With a more reasonable 50% compaction, this would need a matrix with a S.G. of less than 1.4 to have neutral buoyancy (which is easy given that most epoxy S.G. is ~1.2). On the flip side, you really should not sand or use microsphere fillers in thin sections. This will damage the microspheres and allow paths for water pressure intrusion (a big thing depending on how deep you submerge or if subjected to freezing).

 

This is very helpful. Cabosil is super light, but in situ with epoxy does not seem so, and this bit perplexed me some, I'll admit.

I generally do not like to finish with the Sil32 and use Quikfair. And we use epoxy primers, so even sanded sil32 ought to be sealed; perhaps but for a pinhole.

Systems Three Quikfair lists a net mixed density of 6.56 with no units on their webpage download. I have an unopened 3 qt kit here that weighs 5 pounds 8.3 ounces in plastic containers... 3 quarts is 175 cuin. So, lets say the material is 5 pounds. 175 cuin is 0.101 cuft. So the estimated density is 5 lbs/0.101 cuft or 49.5 lbs/cuft. Converting to g/cm3 is 0.016 or 0.792 g/cm3

49.5 lbs/cuft converted to per gallon is 6.62 versus their figures, so it appears they are using pounds per gallon as their density.

6.56 pounds per gallon is 49.07 pounds per cubic foot; so just a little contribution, sorry for all the painful calcs

Seems like a fairing compound ought to be like 30 pounds per cuft to me..

 

Cab-o-sil as packaged is mostly air...and all that volume is going to get filled with epoxy.

I'm pretty sure the webpage is in lbs/gallon. A gallon of water (fresh) is 8.34 lbs give or take. So that makes the S.G. 6.56/8.34 = .786 or ~50 lbs/cuft which is reasonable. 30 lbs/cuft non-cellular epoxy matrix foams are very difficult to make and require marcospheres, not microspheres.

 

You may want to look at these non-hygyroscopic foam suppliers...:

Search non hygroscopic | SAI INSULATION INDIA 9738386050, Bangalore https://www.saiinsulationindia.com/search/-non-hygroscopic/3
https://www.rockwool.com/contentass...m-brochure-26.08.2021-en.pdf?f=20210907102544
Knauf Earthwool Insulation Batts https://jamesbuildingsupplies.com.au/products-and-services/knauf-earthwool/

These maybe suitable alternatives..

 

I think what fallguy is looking for is a trowelable fairing compound like that used for fillets before glassing or just fairing large areas or seams because he quotes System 3 Quickfair. I'm pretty sure that it will be tough to formulate a fairing putty that light that can be spread that thin.

 

I also have clean corecell grindings from 4#/cuft foam. Those would also work for a first pass with some sil32 and epoxy and help push me closer to that 30# dream. But I am mostly surprised the industry data is so hard to find.

 

I think the data is hard to find for home mixed values of density because the vendor has such little control over it. Really, it is a problem of sphere packing (Sphere packing - Wikipedia https://en.wikipedia.org/wiki/Sphere_packing#:~:text=Recent%20research%20predicts%20analytically%20that,will%20yield%20a%20regular%20packing.) Even with polystyrene expanding foam, how you handle the expansion can greatly very the actual density, often by a factor of 10.

 

The lightest fairing filler that I know off is the Wast Microlight, followed by Phenolic microbaloons and then the mineral microspheres like your sil32.
There will not be a significant difference between the three in the mixed bog density. It will be around 0.5 to 0.7 g/ml mostly due to personal recipe, I have never measured this just an estimate.