West Epoxy with Colloidal Silica - question about consistency

[jsargevt]

Is it normal to have a bumpy set of external fillets when you mix in C.Silica? I mixed that batch for what seemed like forever and I had two issues.

Background: I marked the tape line with the 1/2" PVC scrap I colored with marker. Mixed up 2 pumps of West and added in Silica until it was peanut-buttery! I mixed an additional minute or 2 hoping that I could make it smoother than last time.

The issues:

1. The fillets were VERY bumpy despite looking smooth after pulling with an alcohol-dipped PVC scrap. Once they cured it was very bumpy.
2. The fillets had some voids that I swear were not there when I pulled the fillets. I wonder if I might have mixed TOO much and added in some air that escaped and then created voids.

Just wondering so my next set look nicer and are not as much of a pain in the tuckus to make smooth.

Bonus question: what is the best way to make transitions into and away from the fins with epoxy? Again, I had bumpy, pinholey sort of triangles that look ugly out of the gate. I also am doing this on a split fin and it seems the area between the split fins is difficult to achieve a nice streamlined look. I suspect practice makes perfect here.

That, or some proline hi temp.

Thanks in advance!

[dixontj93060]

How long did you mix the batch? I can tell you from experience you need to beat the living daylights out of it--I'd say 3 to 4 minutes at 120 beats/sec.

[jsargevt]

How long did you mix the batch? I can tell you from experience you need to beat the living daylights out of it--I'd say 3 to 4 minutes at 120 beats/sec.

120 beats a sec? I think you could start a black hole with that!

I didn't do it that long. I will next time. I wonder if a Popsicle stick in a drill would work since I'm lazy.

Thanks for the info!

[dixontj93060]

120 beats a sec?

Oops, 120 beats per minute...

Personally though, I have pretty much using colloidal silica for external fillets because of its finish inconsistency. I use the SM Fairing Compound from US Composites found on this page.

[SSenesy]

In my experience, colloidal will always give a bit more 'grainy-ness' than something like the micro-light filler. Further, since it tends to be firmer, it doesn't seem to 'flow' out like some of the other fillers or straight epoxy.

When I use colloidal, I do the best I can to get it smooth going down, but don't worry about it too much. Once the epoxy has dried, I go over it with 60 or 100 grit and knock down the high spots. Then I use Bondo Professional Finishing Glaze (the 2 part filler, not the 1 part) to fill in the voids. Just use your gloved finger to fill everything in, then sand it out when dry. MUCH easier to sand than colloidal.

Just my $.02 but it works for me.

Here's a pic of a Hawk that I've done colloidal on, then put the glazing putty down, but it hasn't been sanded out yet. The second pic is of my Extreme Darkstar done the same way (airframe gets painted black tomorrow).

[neond7]

I have been using West System with Colloidal for all my fillets - and yes, they do tend to turn out a little bumpy for some reason. I always get a fairly straight sharp raised edge where I pull up the tape, since its so thick it doesn't "flow" and self level once the tape is removed. Because of this, once the epoxy cures, I use Bondo spot putty and put a few thin coatings over it and then let it dry. After the Bondo dries, I sand it smooth and then use more Bondo to fill any additional spots that need more work. After a few rounds of that, it turns out looking great. Never had any cracks or issues and the Bondo allows a nice smooth radius between the fin and the body tube.

The first two pics are from the last two builds I did. You can see the fillet is a bit rough. No craters or anything, but not real smooth. I swear by the Bondo, it sands like a dream. In the last pic, I tried to use it to fill in the spirals, but I don't think it worked that good. I'll try the Elmers wood filler that several people have recommended. But for fillets, Bondo is your friend!!!

[mkadams001]

If you are using 406 you may be mixing it too thick, pull it back to around mayonnaise or slightly thinner. When mixing add the thickening agent a little at a time until you get the consistency that you want. I use 406 all the time and don't have the problem that you describe.

If you are using epoxy to create a transition try using west systems 410. It is super light and easy to sand.

Could you post some photos of the problems that you are having with the fillets and transitions?

[Pat_B]

One thing that helps a little is to initially mix your silica with either the resin or hardner by itself. It stirs in easier as compared to mixing into the resin/hardener mix, and also gives you more time before it starts to cure.

[jsargevt]

Thanks all.

1. I have discovered the 90 grit to knock down then smooth with bondo trick and that does make it a lot easier to deal with and produces a nice result.
2. I do not have any pictures of the fillets since I invoked item #1 above yesterday on my day off but if this happens again I'll take some pictures for sure.
3. I was told to mix resin & hardener THEN add in your fillers. I could try it the other way but it would have to be resin > filler > hardner since there is so little hardner used in west.
4. I think that I AM using too much 406 silica. I'll make it less goopy and maybe I'll get some of the "flow" that will help minimize tape lines as well.

Thanks all for the diagnostic! If there are some ugly spots left after today's second-round bondo sand I'll take some snaps but they were looking much better yesterday after the first bondo sand.

[jsargevt]

Just a follow up to the thread:

I was not mixing the epoxy long enough. I did up a 3 pump batch for my arcas and used a spoon to mix for 3 minutes and it came out perfectly.

Of course I had a different issue. Everything was going awesome until I got to the backside of the third fin. I tried to scoop out a glob of epoxy and I got a gigantic chunk of epoxy. There were areas of workable epoxy but the big goo ball in the middle was starting to take over! I hurried to get the last fin done but the epoxy was very chunky by then. I used my finger dipped in alcohol (gloves on of course) and smoothed it out as best I could. It was a bummer to lose all that epoxy as I had a few other things that I was going to glue. I assume that the pot got hot and started curing on me a little earlier. Alas....learn something new every day!

Mercifully it sanded down well with some 80 grit and it is currently sitting with bondo on it waiting for sanding session #2 of many.

I wish that I knew how much sanding was involved in this hobby before I caught the bug. I might have done something cheaper like car racing or sailing (kidding kidding).

Thanks for the suggestions all.

[Pat_B]

Put a drop or two of red stamp pad ink into your mixture. It'll make it easier to see when the mixture is fully blended.

The benefit to mixing the filler with the resin first is that you aren't losing time with your mixture curing while you do the necessary stirring to get the filler blended in. You can take your time, then add the hardener.

[cwbullet]

Put a drop or two of red stamp pad ink into your mixture. It'll make it easier to see when the mixture is fully blended.

Or epoxy coloring.

[G_T]

Once mixed, pour out the epoxy onto a disposable plate. West exotherms pretty radically. In a thin layer, it doesn't cook itself off so fast.

Gerald

[Pat_B]

I've been using a lot of epoxy all summer and I always have to be careful where I place the extra that's left over. I try to remember to put it on the floor in its little cup so that it can't start anything on fire.

[jsargevt]

I did forget to do filler + resin first. I suspect i did lose a LOT of time due to adding in the filler slowly so not to overdo the filler. I could feel the cup getting warmer so I was suspecting that I was on my way to an early cure. It didn't feel like it was getting hot enough to start a fire though. That would extremely bad! I'll note that for the future.

I also like the idea of a bit of color - I can swing that I am sure.

Thanks for all the tips!

[dixontj93060]

Better not to use a cup--too contained. Dixie makes a nice line of paper plates with high lips which I have been using lately. If you want to use a cup for measuring purposes then just transfer to plate (or bowl) to use during application.

I did forget to do filler + resin first. I suspect i did lose a LOT of time due to adding in the filler slowly so not to overdo the filler. I could feel the cup getting warmer so I was suspecting that I was on my way to an early cure. It didn't feel like it was getting hot enough to start a fire though. That would extremely bad! I'll note that for the future.

I also like the idea of a bit of color - I can swing that I am sure.

Thanks for all the tips!

[cwbullet]

120 beats a sec? I think you could start a black hole with that!

I didn't do it that long. I will next time. I wonder if a Popsicle stick in a drill would work since I'm lazy.

Thanks for the info!

That is funny. I use a cup to mix it and then spread it out on an open surface to avoid premature hardening.

[G_T]

If you mix the filler with the epoxy first, you are very unlikely to get a good mix. You cannot mix as rapidly. You cannot tell visually for the first signs of sufficient mix. To get to the point of an adequate mix, you'll end up taking roughly the same total time. But I'd bet people aren't doing that...

Mix the epoxy first. Mix until the appearance goes glass clear. Then mix for that much additional time. Additives go after. This is pretty basic composite stuff. In cases where you really care about the structural properties, transfer the epoxy to a new container and use a new mixing implement halfway through the mixing process. Scrape during the initial and second mixing, but NOT during the transfer. That helps assure that the resulting plastic has uniform physical properties. Also keep a sample until cured, so you can check that nothing went wrong.

If you need to really extend your working time, use a slower hardener. Or lower the temperature. Each 10 degrees C probably doubles your working time. Spread out into a thin layer. Particularly with epoxy systems which exotherm strongly (West, System 2000, System III, etc) this increases the surface area for cooling and reduces the self-insulation, thus keeping the temperature down and the working time up.

I have a part which I manufacture which requires 2 hours of working time at room temperature. I use two batches of epoxy, the second one at 1 hour. With the epoxy I use, this is not a big deal. At the end of the process the first batch is thickening but still workable.

Part of doing a good job is choosing appropriate materials, and using the correct procedures to assure success.

Be aware that West Systems has a very low glass transition temperature and there is nothing you can do about it. Post-cure won't help with this system. So if one is out in a hot desert and expects the fillets to be solid, that won't be the case. On a hot day with the sun directly on the fillets, the epoxy will have passed out of the semi-crystalline phase and gone into the plastic phase (T > Tg). One could apply steady pressure and change the alignment of the fins. Just FYI.

Gerald

[AndyC]

If West kicks off too fast for you, try Aeropoxy. Its really great stuff, and can be post-cured (though I never have...). I rarely spread it on a plate or other flat surface to avoid kicking off with the med hardener(30-40min), and never had a problem. The only time I do it is when laminating, when there is much more epoxy mixed up and I need more time - then I use these ~5" plastic dishes from the local takeout Chinese place, makes it easier to get a paint brush in it too.

I've had bigger problems with hobby 30 min epoxy kicking off in a cup, even in <1 oz qty. A tip there is to put the cup on a block of blue ice, or whatever sealed ice pack you happen to have in the freezer. Works great, and less loss than spreading it over a paper plate.

[cwbullet]

I've had bigger problems with hobby 30 min epoxy kicking off in a cup, even in <1 oz qty. A tip there is to put the cup on a block of blue ice, or whatever sealed ice pack you happen to have in the freezer. Works great, and less loss than spreading it over a paper plate.

This is true. In 90-100 temps, you can hald you work time.

[troj]

In my experience, West Systems epoxy is much more sensitive to ambient air temperature than AeroPoxy and Fibreglast. It's part of why I quit using it, to be honest.

Mixed and applied properly, you shouldn't be getting the bumpiness. My guess is you're going voids underneath and the fillets are slumping a bit as the epoxy cures.

Pat's idea of a trace dye amount to help verify you're getting it fully mixed is a good one. Though the recommendation for epoxy dye is better, as you know it's not going to affect the cure.

-Kevin

[JimJarvis50]

When I use the coloidal silica, i have to mechanically break up the larger globs of the silical to get it to wet out. I just use a mixing stick against the side of the mixing container to do that.

One of the main reasons for using the silica in the first place is that the viscosity of the mixture has some interesting characteristics. It forms a thixotropic or non-newtonion fluid where the viscosity is affected by how the mixture is being sheared. When you mix it or apply it or shape it (i.e., shearing the mixture), it has a lower viscosity. But, when there is no shear, the viscosity is higher. Thus, you can use it for your fillet because when you stop messing with it, it won't run. The reason I mention this is that perhaps you're making the mixture too thick and not taking advantage of the properties of the material.

Jim

[troj]

Something I discovered yesterday, while working on fillets, made me think of this thread.

What did I discover? Shape Matters!

Specifically, the shape of the container you're doing your mixing in.

We have a pile of the little "boats" that Buffalo Wild Wings provides for when you buy large batches of wings. I decided to use some of them to mix epoxy, and did a batch for fillets on my Talon 6. I found that not only was mixing the epoxy a pain in the keister (the hardener is a different color than the resin, so on the white background it's easy to see when they're not mixed), but mixing in the filler (fumed silica) was even worse.

My second batch for the day, I went back to my standard -- yogurt containers. It's much faster and easier to mix in the small, round container than in that darned boat. With the boat, I had to manually break up the filler clumps; in the yogurt container, just mixing took care of them. In the end, I had the same result, but one was a lot more work than the other!

-Kevin