Help with Fillets!

[Jonsayler]

Hello everyone. I am completing my first 4" fiberglass kit. The fin slots seemed to have been cut a little wide making the fins quite loose. To accommodate this I made pretty large fillets for the fins. I overdid it now and they look absolutely terrible! Any ideas? Can I sand them all the way down and do them again? There are ridges and divits and its a complete mess. I used rocket epoxy and that stuff is terrible to sand my hand. Thanks everyone

 

[kenstarr]

Post a picture please. What im imagining suggests you could use a dremel type tool to grind them down somewhat and smooth on a new layer. Hand sanding will involve a coarse grit paper and a rod of your desired radius. I've done some ugly fillets in the past before learning techniques to make them really pretty.
Ken

 

[Hobie1dog]

a dowel rod with 80 or 100 grit paper adhered to it will knock down an unwanted fillet pretty quick.
put some heavy/aluminum duct tape up on the fin and airframe so that you don't dig into them.

 

[teepot]

I was going to suggest a Dremel too.

 

[Hobie1dog]

you'll have to be very careful with a Dremel.

 

[David Schwantz]

Heat up the epoxy with a hair dryer, go slow. you will then be able to flake it off in chunks, sand the area again and just start over.

 

[Jonsayler]

 

[neil_w]

This is a good opportunity to review basic filleting techniques, most notably the use of masking tape and a curved object to pull the fillets smooth. Your next ones will be better!

 

[Jonsayler]

This is how bad it it. It’s embarrassing. I made the terrible decision to try and use bob smith epoxy on top. Logic being it’s thinner and I could smooth it out easier but I actually made the situation worse. I would like to lay down a new rocket epoxy layer but I don’t want to made any mechanical connection weaker as well. Id rather have an ugly fillet then a broken wing.

 

[David Schwantz]

Once again, heat will take that off no problem. Get a plastic scraper of some sort, even a plastic knife or spoon will work. Heat up the glue and scrape it off. the stuff will just roll off. And just start over.

When doing the new fillets, the tape off method works great, but I prefer applying the glue with a syringe in just the right amount and then pulling it with my finger. Hardly no sanding afterwards. I does take practice though. Try it on a scrap piece first.

 

[mtnmanak]

All good suggestions here. My take on it is that I stopped trying to do nice, smooth fillets on HPRs while the epoxy is wet a while ago. I glop an epoxy/chopped CF mix on the fillets nice and ugly. Get a bunch on there and really let them kind of soak in. Then I take them down quickly with a random orbital sander, fine sand with a round Dura block, and really fine sand with a dowel. Then bondo it all to fill gaps followed by a final fine sanding to make it all smooth and pretty. For me, this method has produced super strong, flexible fillets that are quicker and easier to get smooth than trying to get it all nice while the epoxy is wet.

Example from an Goblin:

 

[teepot]

I use PC Super Epoxy. It doesn't run, drip or sag. It stays where you put it. I can do 4 fins at the same time. First I mask the area where the fillet will go. Then using a popsicle stick sized craft stick I put the epoxy down. After 10 minutes I pull the fillet with a tongue depressor sized craft stick. Wait 10 to 15 minutes then remove the tape. If it isn't as smooth as you want. Spray some alcohol on the fillet and smooth with a finger.

 

[David Schwantz]

These are fillets that were done with the tape method and syringe just pulling with my big fat finger afterwards. No sanding. Proline epoxy from Wildman.

 

[DragonRocketry]

30-minute epoxy, micro balloons and tongue depressor. Tape is your friend.

 

[dr wogz]

we all have our own technics & tricks, and preferences. But ultimately, it comes down to patients & practice..


(I'm also amazed at how many 'think' they can clean up a fillet after it's set & cured. wipe up / clean up / scrape up the stray globs when they are wet, not when dry. It's easier to sand smooth a minor smear, than a mountainous glob.. especially when you throw in all the filers & strengtheners... and clean / wipe you tools between pulls & fixes!!)

 

[roytyson]

Here is a white paper I made for how I do mine.

 

[cavecentral]

I used a half-round file and using the rounded end did basic shaping fairly quickly. Not as easy to screw up as power tools, but much faster than sandpaper.

 

[REK]

View attachment 538095

Those fillets don’t look too large in diameter. I’d recommend sanding with an 120 or 180 grit to get rid of the bumpy spots. Then with a slightly bigger diameter wooden rod. Make new mark lines with carbon paper and then apply masking tape on the mark lines.

Apply epoxy to the area as some have shown on here how they do it and using the same dowel to make the new fillets. Depending on the epoxy your using. Always give it some time after mixing to allow it to gel and this way the epoxy doesn’t run.

 

[Pooklord]

I was going to suggest a Dremel too.

Dremel is great because it can remove material so quickly, but that also is the danger! I like to use it for bigger rockets and hand sanding for lpr rockets.

 

[Jacktango22]

Is the primary purpose of the fillet to add to the structural integrity of the fin or to help with airflow where body and fin connect? I am back into this after decades and studying and learning from the vast trove of information available on the web.

thanks

 

[David Schwantz]

Both.

 

[JRL303]

Masking tape of course; previous post demonstrates beautifully. I typically do a smallish epoxy fillet for bonding and finish with a top coat of Poly-Fiber Super Fil for cosmetics. Very easy to sand and smooth to a nice finish.

 

[JLP1]

YouTube Bad Assed Rocketry, he has a 2-1/2 minute video on his site for making almost perfect fillets using rocket poxy, tape, and those metal fondue balls. A little practice and you can make some really nice fillets. One of the vendors recently bought out BA or he bought them out I can't remember but the videos are still out there. Works on just any type of tube.

 

[Tractionengines]

Once again, heat will take that off no problem. Get a plastic scraper of some sort, even a plastic knife or spoon will work. Heat up the glue and scrape it off. the stuff will just roll off. And just start over.

^^^^This. For reference here is a rocket that broke a fin on a hard landing. It had 1" wide fiberglass straps in the fillets. A few minutes with the heat gun, and pliers, and I peeled the fillet, and fiberglass strip, right down to the original tube.

 

[waltr]

YouTube Bad Assed Rocketry, he has a 2-1/2 minute video on his site for making almost perfect fillets using rocket poxy, tape, and those metal fondue balls. A little practice and you can make some really nice fillets. One of the vendors recently bought out BA or he bought them out I can't remember but the videos are still out there. Works on just any type of tube.

Video is on Mach 1 web site here:
https://www.mach1rocketry.com/badass-videos
On my first epoxy fillets I followed this video and had perfect fillets. Actually very easy to do.

 

[techrat]

If you have a ROUND file, that could smooth out the fillets before you get down to sanding. However, what you've got is salvage-able if you take your time and use some sandpaper. Since your rocket is fiberglass, you can do wet-sanding too and make it a little more pleasant. But start with a file to take off the high spots, then 80-grit, then 150, then 240 then 340 and then 600 and you can make it look nice.

 

[Zeta]

These are fillets that were done with the tape method and syringe just pulling with my big fat finger afterwards. No sanding. Proline epoxy from Wildman.

I once had a girlfriend that was almost as beautiful as those fillets are.

~Zeta