Fairly rough finished surface with epoxy and colloidal silica

[WizardOfBoz]

I used West 105/205 epoxy with West 206 colloidal silica for my fillets. Some observations:
1) Using any amount except the extreme minimum for a joint is like building a rocket with maple syrup.
2) On must add enough silica to really get the stuff thick so that it doesn't run

Regarding point 2, I mixed up a "peanut butter" thickness and used popsicle sticks to form the fillet. What resulted was a really rough, dimpled finish. It will be fine with sanding, but I've questions.
1) What is the normal finish that results from epoxy/silica?
2) Did I use too much 206?
3) Do folks sand down the dimples all the way, or do they use a filler like clear-coat (105/207) to fill in the dimples? Or some other filler?

Thanks.

Pic of filleted Madcow Super DX3 and Estes Argent (unbuild Eggtimer DD rocket on left).

 

[WizardOfBoz]

Bump. No comment? I'm trying to find out if I put too much colloidal silica in my 105/205 mix. Thx.
\

 

[manixFan]

In my experience with using most fillers, there will be some bumpiness. It's probably due to the fluid epoxy settling and tiny clumps of filler then standing proud of the surface. It's possible that more thorough mixing would help mitigate the effects. I've always just sanded the surface smooth – most fillers sand quite easily. If I'm really going all out, I may smooth the surface with a finish coat of epoxy, but generally I just use Bondo spot filler putty (the red stuff in a tube).

Keep in mind that many fillers actually weaken the overall strength of the epoxy, but typically add flexibility, which may be a decent tradeoff.

I have used chopped Kevlar as a filler. That stuff is unsandable! For that I cover with another coat of thickened epoxy and sand that.

Good luck,


Tony

 

[Steve Shannon]

To ensure that I have just the right viscosity I add the silica to the hardener and resin separately. Only when each of the components (resin or hardener) are completely mixed with silica, and I’m ready to use the epoxy, do I combine the resin to the hardener. That allows me to ensure that I have mixed in the silica completely without working against the pot time.

 

[heada]

I do a minimal epoxy fillet with milled fiber to act as the structural joint. Once that cures, I sand it with rough sandpaper, normally 80 grit, and then add a thin cosmetic fillet on top of that made with phenolic spheres (aka micro balloons) The cosmetic fillets sand really smooth and get to a feather edge.

 

[ECayemberg]

Hi Wizard,

Agreed that 105/205/Silica leaves a rough surface. That said, it's still a good choice for lasting fillets.

What I do to take care of the bumpiness....I don't worry much about sanding the filled epoxy itself. Add one layer of red glazing putty; usually applied with a gloved hand. Takes maybe one minute per fillet wiped on. Let dry, sand with a round dowel with sandpaper over it. Result is a strong, smooth fillet with minimal effort. Generally, requires only 1 round of filling and sanding.

 

[alexzogh]

Ding ding ding…. Eric nailed it. Glazing putty or Bondo makes a great surface for paint.

 

[prfesser]

I thought I'd posted on this, sorry.

I mix Cab-o-Sil or the generic version very thoroughly into the resin first. It should be virtually smooth, takes a lot of mixing. For larger amounts, coathanger wire bent into a mixer-blade shape and chucked in a drill is great. Then I add curative. That thins it out somewhat, may take a little trial and error to get it just like you want it.

Most thixotropic epoxies are prepared in a similar manner.

 

[WizardOfBoz]

Thanks to all. Lots of good info. I suspect that I didn't mix the silica in enough, even though it looked pretty uniform.
105/205/406 IS hard to sand but it's doable. Someone on this site has a sig file that says "80% of rocketry is sanding". With this mix, 90%.

Eric, I'm ordering the red glazing compound. In future builds I may order the microballoons (assuming the stuff I found here is what is being refered to).
Great tip to mix the silica in before the initiator. This will give me more time to work with the stuff.
The replies ended up leading me to John Coker's tips (here) about using a plastic spoon and somewhere else I read a recommendation that one can wet the spoon/glue surface with a little IPA. This aligns with experience with latex bathtub caulk - one can use a finger, wetted with water, to get great caulk fillets.

BTW, ECayemberg what rocket design is that? Very appealing, and nice job on the fin profiling. (on Edit: I did a bit of interweb sleuthing and suspect that the rocket was a LOC Force Ten. Is that correct?).

 

[terryg]

Fondant Cake Pan Flower Decorating Clay Flower Sugar Craft Ball Model DIY Tool (Small (4pcs))

https://a.co/d/bQvq1Fh

 

[WizardOfBoz]

@terryg, thanks for this. - Very cool repurposing of the cake decorating tools. And my Grandfather was a commercial baker (so i always had REALLY fancy birthday cakes growing up).

 

[WizardOfBoz]

I mix Cab-o-Sil or the generic version very thoroughly into the resin first. It should be virtually smooth, takes a lot of mixing. For larger amounts, coathanger wire bent into a mixer-blade shape and chucked in a drill is great. Then I add curative. That thins it out somewhat, may take a little trial and error to get it just like you want it.

Most thixotropic epoxies are prepared in a similar manner.

prfesser, your answer kind of underscores what I suspected: I'd not mixed the stuff up enough. Also, as others have said, the stuff often goes on a little rough.

I really like how you mixed the practical (coat hanger mixer) with the theoretical mention of non-newtonian fluids. Thixotropic is not a word used in normal conversation!

 

[cls]

Thixotropic is not a word used in normal conversation!

It is, around here!

 

[WizardOfBoz]

It is, around here!

I'm trained as a chemical engineer. It's been a long time (undergraduate studies are a distant memory) since I used word and phrases like thixotropic, dilatant, and Bingham plastic. Transport Phenomena class and Newton's law of viscosity. Those were the days...

 

[WizardOfBoz]

Lowes had Bondo red glazing compound. Progress so far:

 

[ECayemberg]

Thanks to all. Lots of good info. I suspect that I didn't mix the silica in enough, even though it looked pretty uniform.
105/205/406 IS hard to sand but it's doable. Someone on this site has a sig file that says "80% of rocketry is sanding". With this mix, 90%.

Eric, I'm ordering the red glazing compound. In future builds I may order the microballoons (assuming the stuff I found here is what is being refered to).
Great tip to mix the silica in before the initiator. This will give me more time to work with the stuff.
The replies ended up leading me to John Coker's tips (here) about using a plastic spoon and somewhere else I read a recommendation that one can wet the spoon/glue surface with a little IPA. This aligns with experience with latex bathtub caulk - one can use a finger, wetted with water, to get great caulk fillets.

BTW, ECayemberg what rocket design is that? Very appealing, and nice job on the fin profiling. (on Edit: I did a bit of interweb sleuthing and suspect that the rocket was a LOC Force Ten. Is that correct?).

Correct. It is/was a quick build to see if unreinforced cardboard could sustain an "N" motor. I knew it could, but was/am happy to help prove it!

 

[WizardOfBoz]

Feedback on Red Glazing Compound. I love it. Got the Bondo version. "Bondo Glazing and Spot Putty"

I've used in on two larger rockets (Super DX3 and Estes Argent) for smooting the fin fillets, and on a smaller Estes Patriot to fill in the body tube gaps (the thing is 21 inches long and has TWO tube couplers!!?) as well as a balsa wood grain filler. Worked great - easy to apply and to sand. Filled the balsa grain in one shot. The larger rocket is primed only - it's been too hot to use rattle cans here. The smaller (Estes Argent) is now yellow (not shown). The key thing is that the glazing compound is the bomb.



I ordered an Estes Nike Smoke. To get the order above $100 (eliminates shipping) it made sense to buy a bit more. The Estes Patriot was my choice and was fun. I give myself a C on the finish, but I learned a lot. The red glazing compound (Bondo was what Lowes had) worked great as grain filler for the balsa fins. Bonding primer to the nose cone after washing, sanding, and rubbing with acetone. Rustoleum sandable primer over everything. All the surfaces were great except I zapped the red ring with a bit too much paint. I live. I learned. Looking forward to sneaking out to launch this next week (I have a flat 7 acre green space next to my house.). OR predictions are aligned with Estes: 300 feet on a B6. 600 feet on a C6. If all goes well, I'll order a some Quest and AT SU D engines... But the key thing is that the bondo glazing stuff filled the grain on the first pass. Thanks for the tip!

 

[bjphoenix]

Feedback on Red Glazing Compound. I love it. Got the Bondo version. "Bondo Glazing and Spot Putty"

There is a one component product that air dries, and a two component product that chemically cures. My understanding is that the one component shrinks a little bit as it dries, the two component doesn't.

We had some paint work done in our house, our painter used the one component to fill in some dents in door trim. The smell was very strong in your house. When my father owned a body shop the guys would use the putty on car repairs. I don't remember if they used one component or two component but I don't remember the smell being so bad.

 

[WizardOfBoz]

The stuff I used was one component that air cured (or dried - whatever). It did have an odor. Can't tell if it shrunk as I applied it over the bumpy epoxy and sanded it down. Again, I thought it worked great.
Thanks, @ECayemberg , great tip.

 

[prfesser]

In my hands the one-part glazing putty shrinks rather less than water-based wood filler. And it dries a lot faster. However, do it outside.