Bondo over epoxy fillets

[Buckeye]

I would like to touch up my RocketPoxy fillets with Bondo, as others have mentioned - fill in depressions, pock marks, and get nicely sculpted fore and aft end points. What is the method? Do you pull another complete Bondo fillet with your radius tool, or just apply the Bondo where it is needed and try to blend it in?

Edit: Yes, I know I need to sand the Bondo to desired shape no matter what!

 

[rharshberger]

Unless the depressions are large, prime then use Bondo Spot & Glazing putty to fill imperfections. Not sure that regular Bondo is ideal for sculpting where the fillets come together on the leading and trailing edges, airframes and fins tend to be a bit flexible and the Bondo is not very flexible.

 

[Banzai88]

If you have large divots, fill with more of the base epoxy that you formed the original fillet from. "Nicely sculpted fore and aft end points" is best accomplished this way, with some judicious planning, taping, and sanding. Bondo alone will not do much for you, and is likely to crack with any flexing. Same with any epoxy putty like Apogee advocates: Cracks easily.

Spot and glazing putty is for filling small cosmetic surface scratches and divots for the purpose of a smooth final paint surface and will NOT stand up to anything like forming it into leading or trailing edge structure.

 

[Buckeye]

Unless the depressions are large, prime then use Bondo Spot & Glazing putty to fill imperfections. Not sure that regular Bondo is ideal for sculpting where the fillets come together on the leading and trailing edges, airframes and fins tend to be a bit flexible and the Bondo is not very flexible.

If you have large divots, fill with more of the base epoxy that you formed the original fillet from. "Nicely sculpted fore and aft end points" is best accomplished this way, with some judicious planning, taping, and sanding. Bondo alone will not do much for you, and is likely to crack with any flexing. Same with any epoxy putty like Apogee advocates: Cracks easily.

Spot and glazing putty is for filling small cosmetic surface scratches and divots for the purpose of a smooth final paint surface and will NOT stand up to anything like forming it into leading or trailing edge structure.

Thanks. Yes, I am using Bondo Spot & Glazing putty #651.

Indeed, I try for judicious planning and epoxy application to get the end points right the first time, but sometimes it doesn't always work out. Maybe some pics will help. In the first pic, I am happy with the epoxy shaping and light sanding. In the second pic, I was hoping to use the Bondo to touch up the highlighted defects.

 

[dr wogz]

Buckeye. in your case, I would apply a bit of Bondo (glazing putty) to the circled areas with a putty knife, Then bland with sand paper.. The leading edge work will likely take you 2 or 3 passes with the putty & sanding to get right..

Then again, I typically use a sandable filler in my epoxy when I make fillets, as the epoxy is then easier to sand & blend to shape & mating surfaces. (no need to 'add' another 'filler')

And I tend to 'round off' my leading & trailing fin edges.. (Sorry, that sounded rude, but isn't intended to be..)

 

[Banzai88]

Spot and glazing putty will stick better over primer than it will on a naked surface

 

[Buckeye]

I am going for this look. Photo courtesy of @rfjustin

 

[Buckeye]

Spot and glazing putty will stick better over primer than it will on a naked surface

Interesting. I know this is true for wood and cardboard, but I didn't think it would hold true for fiberglass.

 

[Banzai88]

Interesting. I know this is true for wood and cardboard, but I didn't think it would hold true for fiberglass.

Ok, but auto shop taught that we use spot putty over primer to get a chemical and mechanical bond, and that it will blend better with the primer when sanding for a final finish

 

[rfjustin]

Ok, but auto shop taught that we use spot putty over primer to get a chemical and mechanical bond, and that it will blend better with the primer when sanding for a final finish

Good info to know, thanks!

 

[manixFan]

I am going for this look. Photo courtesy of @rfjustin

View attachment 485833

I’ve used the glazing putty exactly like that, normally with great results, on both bare surface and over primer. I’ve found doing it over a coat of primer makes it a lot easier to see the contours. However, on my MD speed demons, after Mach 2 there seems to be enough heating that causes portions of the putty to basically just pop off. It’s happened on both my 38mm and 54mm rockets, generally at the front taper, but I’ve also had it occur at the rear taper. But for rockets that stay under Mach 2 it doesn’t seem to be a big issue, at least in my experience. So now I use filled epoxy for all fillets for rockets intended to go well past Mach.


Tony

 

[Buckeye]

I’ve used the glazing putty exactly like that, normally with great results, on both bare surface and over primer. I’ve found doing it over a coat of primer makes it a lot easier to see the contours. However, on my MD speed demons, after Mach 2 there seems to be enough heating that causes portions of the putty to basically just pop off. It’s happened on both my 38mm and 54mm rockets, generally at the front taper, but I’ve also had it occur at the rear taper. But for rockets that stay under Mach 2 it doesn’t seem to be a big issue, at least in my experience. So now I use filled epoxy for all fillets for rockets intended to go well past Mach.
Tony

Good to know. This is a 54mm MD Wildman Mach 2, LOL! You all saw what happened to the plastic printed fin can I used last time on this rocket.

I have a 2 or 3 day road trip in front of me if I want to push this rocket that hard, so it may be a while. I can enjoy it many times at lesser speeds before then.

 

[Buckeye]

My best fillets ever. Hope the Bondo doesn't fly off at Mach 2.

 

[manixFan]

My best fillets ever. Hope the Bondo doesn't fly off at Mach 2.

View attachment 486582

I hope SEXY! isn’t inappropriate here! Let us know how they hold up.


Tony

 

[6inchmonster]

I used to do several passes of bondo spot&glaze putty (or whatever the proper name is) over extra chunky west 406 fillets, holding up great over time and even a near-MD M3700 screamer that should have been around mach 2 (baro data only sorry. I do not like the feel using it with gloves, but then I do not think it is great to have on naked skin either. In any case I mostly do proline fillets now, but more because of the mechanics than the performance of my old fillets.

 

[Steve Shannon]

I used two part bondo to fill the weave and clean up fillets after fiberglassing my 7.5 inch level 3 rocket more than 16 years ago. For sentimental reasons I’ve only flown it a few times since, but it has been temperature cycled and rattled around in my truck on the way to classrooms many many times. The bondo has withstood the test of time. It’s on top of aeropoxy if that’s a factor.

 

[Bowman]

Ok, but auto shop taught that we use spot putty over primer to get a chemical and mechanical bond, and that it will blend better with the primer when sanding for a final finish

Just a note: Spot or glazing putty is basically thick primer (like toothpaste) and will adhere to any well prepared surface like primer does. It is also fairly brittle when dry so large (thick) amounts will tend to crack with any flexing.

 

[jbsommerfeldt]

Aircraft supply. My dad used in his plane. I used in my rockets. If you ever flown in small plane they shake, rattle and twist in all kinds of crazy ways. However, Cesena 152 probably only going 150 mph. But I have not had a Super fil just "pop off". I would NOT make an entire fillet out of this stuff. But great for after main fillet then this to smooth it out. I bet every lvl 3 rocket I have has this on it some where. Nose cone, body, dented fiberglass, etc......

 

[mtnmanak]

View attachment 487432

Aircraft supply. My dad used in his plane. I used in my rockets. If you ever flown in small plane they shake, rattle and twist in all kinds of crazy ways. However, Cesena 152 probably only going 150 mph. But I have not had a Super fil just "pop off". I would NOT make an entire fillet out of this stuff. But great for after main fillet then this to smooth it out. I bet every lvl 3 rocket I have has this on it some where. Nose cone, body, dented fiberglass, etc......

How easy is this stuff to sand?

@Buckeye - I know you already completed this fin can, but in answer to your original question, I personally like to cover everything with the Bondo that needs it and then sand down to the surface. I don't try to "spot" touch up - I go for the whole area. For small rockets, I do the sanding by hand. When it gets up to the big stuff like an 8" diameter rocket, I don't skimp - I pull out the Makita two-handed orbital sander and go to town. I touch up with a Dewalt palm orbital sander.

The gamut:

A minimum diameter 29mm "dart": Fillet, glass, sand, prime, bondo, sand, prime, paint:









To an 8": Same sequence without the glassing.

 

[kenstarr]

I've done bondo over epoxy for cosmetic reasons. It's like makeup for the rocket!
Potentially better yet, get some epoxy filler like fumed silica or microballoons and make your own fairing compound with the same epoxy. Over the years I've gotten pretty good at masking and pulling fillets so I usually don't do much filling anymore. But it seems like I still use a lot of glazing putty.

 

[AfterBurners]

My best fillets ever. Hope the Bondo doesn't fly off at Mach 2.

View attachment 486582

sweet!!!

 

[jbsommerfeldt]

Bondo hates bangs, poke and hard landings. Use bondo, I do too. You see rjjustin ref, I bet 10000 platinum and a faringe ear rub, its similar to super fil. You making museum rocket you fly once or twice? Bondo under straight flight, little to no flex, will be FINE!

At an M pad distance, 300mph in 3 sec, apogee 14000, doing 985 mph, nobody is going to notice your bondo job.

 

[jbsommerfeldt]

I've done bondo over epoxy for cosmetic reasons. It's like makeup for the rocket!
Potentially better yet, get some epoxy filler like fumed silica or microballoons and make your own fairing compound with the same epoxy. Over the years I've gotten pretty good at masking and pulling fillets so I usually don't do much filling anymore. But it seems like I still use a lot of glazing putty.

Just noticed your question about sanding. Um,,,,,,,,, do you like sanding? Everything needs sanding. Compared to bondo it probably sands the same. Heres the thing, let the sand paper do the work. Most people dont under stand that. And it cant be teached. Get it teached, not learned. Its a craft. Use what you got and lots, lots, lots of patients. yes I kill every patient that doesnt do my sanding, I hate sanding. LOL

 

[jbsommerfeldt]

How easy is this stuff to sand?

@Buckeye - I know you already completed this fin can, but in answer to your original question, I personally like to cover everything with the Bondo that needs it and then sand down to the surface. I don't try to "spot" touch up - I go for the whole area. For small rockets, I do the sanding by hand. When it gets up to the big stuff like an 8" diameter rocket, I don't skimp - I pull out the Makita two-handed orbital sander and go to town. I touch up with a Dewalt palm orbital sander.

The gamut:

A minimum diameter 29mm "dart": Fillet, glass, sand, prime, bondo, sand, prime, paint:

View attachment 487540
View attachment 487541

View attachment 487543
View attachment 487544
View attachment 487545
View attachment 487546

To an 8": Same sequence without the glassing.
View attachment 487547
View attachment 487548
View attachment 487549
View attachment 487550

BTW: very cool pix. Better transport that in a sock. Non mil agency might think your legit bomber.