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Body Filler advise?

Mort04

Member
I’ve done a bit of research but can’t find a concusses. What are people using to fill dents and also to blend the line between their fiberglass tips and the aluminum skin. I’ve got 1 dent on the skin of my elevator that I would like to fill as I blend the fiberglass tip once it’s installed.

Thanks for the help
 
Epoxy with micro or something like aeropoxy light. Don’t use bondo or other polyester fillers.
 
I know folks that are using Poly-Fiber Super-Fil. Epoxy based, I think it's mostly micro-balloons. Not cheap, but convenient and comes in that cool blue color, ha ha. I'm sure DanH has his own formulation that duplicates it.
 
Evercoat

Contrary to post 2 on this, it is perfectly fine to use a filler such as Evercoat Rage or if a very thin layer is needed, Evercoat 416. The key is making sure it is a catalyzed filler and the surface is prepped properly.

I used 416 as the fine fill on all of my glass work on my tail glass and other areas. As with any filler, the key is prep. Scuff throughly with 220 paper, clean with alcohol or acetone and apply. Block sand and use a high build primer with a guide coat. That simply means, dust a dark color over the primer. Wet block sand and any low areas will remain dark. Those can be filled with either the filler or if real thin, a concentrated hit of primer. Wet sand until satisfied.

Micro works but can be harder to get that perfect blend that the 416 provides.
 
Evercoat has worked excellent. At the advice of a friend, I have used Evercoat 425. Found it easy to use and excellent filler. Not sure of what the difference is between 425 and 416 other than dollars.

A few years ago, Amazon had the best price BUT they were shipping without hardener. That will add a few dollars to the cost and Amazon price then became the same price as everyone else.
 
Thank you

Thank you for your replies, I’ll take a look at the options you guys laid out. So many options is good and bad haha. Like my tech counselor said “are you building a show plane or a go plane?” I’m trying to do a little of both, but sounds like I need to just pick a filler and go with it.
 
Filler

Thank you for your replies, I’ll take a look at the options you guys laid out. So many options is good and bad haha. Like my tech counselor said “are you building a show plane or a go plane?” I’m trying to do a little of both, but sounds like I need to just pick a filler and go with it.

My choice is West System Epoxy mixed with 410 filler. It’s s the easiest to sand and will hold up well. The epoxy can be mixed with many various fillers depending on the strength you need. Including flox when you need the strength.

If you buy West System epoxy. Make sure you buy the pumps that install on the container for easy, accurate mixing ratios.
 

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If you buy West System epoxy. Make sure you buy the pumps that install on the container for easy, accurate mixing ratios.

From my experience, the pumps are great when new, but start to "burp" or "sputter" a bit after sitting on my shelf for many months - making the mix ratio questionable, especially for small batches. I moved to using a cheap 0.1g resolution digital scale (lots on Amazon) and now mix my epoxy by weight (5.19 : 1 for the West fast hardener I'm using). Very easy to use and never had a bad curing batch!
 
Epoxy callculator

From my experience, the pumps are great when new, but start to "burp" or "sputter" a bit after sitting on my shelf for many months - making the mix ratio questionable, especially for small batches. I moved to using a cheap 0.1g resolution digital scale (lots on Amazon) and now mix my epoxy by weight (5.19 : 1 for the West fast hardener I'm using). Very easy to use and never had a bad curing batch!

My Epoxy Calculator link is on my blog. I always mix by weight. I pour epoxy and hardener into chemical bottles. Condiment containers work but they crack. Chem bottles hold up. https://www.amazon.com/dp/B0816ZRBBP?ref=ppx_pop_mob_ap_share
Makes it easy to dispense exact amounts with zero waste.
 
I do the same thing! I get those bottles from Harbor Freight.. I thought they were a third of the price.. I use a $11 Amazon scale. Mix the epoxy really really good, then add micro. So easy to sand. I would not use bondo if you can help it. Bondo is heavy, stinks and may shrink and crack out, leaving you to do the job twice. Once you get a few epoxy supplies, fiberglass, micro, flox.. you’ll find all kinds of uses!
 
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I do the same thing! I get those bottles from Harbor Freight.. I thought they were a third of the price.. I use a $11 Amazon scale. Mix the epoxy really really good, then add micro. So easy to sand. I would not use bondo if you can help it. Bondo is heavy, stinks and may shrink and crack out, leaving you to do the job twice. Once you get a few epoxy supplies, fiberglass, micro, flox.. you’ll find all kinds of uses!

While epoxy micro is a fine choice i just don’t understand all this dis-information on poly products. Weigh a gallon of evercoat rage and a gallon of epoxy plus activator. Definitely similar in weight . Millions of cars are repaired each year using poly fillers without cracking. I have used it on two RVs with NO cracking. Yes, poly can shrink a small amount, but in most cases it would require a microscope to see it when applied in realistic depth hs.
 
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I

While epoxy micro is a fine choice i just don’t understand all this dis-information on poly products. Weigh a gallon of evercoat rage and a gallon of epoxy plus activator. Definitely similar in weight . Millions of cars are repaired each year using poly fillers without cracking. I have used it on two RVs with NO cracking. Yes, poly can shrink a small amount, but in most cases it would require a microscope to see it when applied in realistic depth hs.

You can’t compare a gallon of bondo to a gallon on epoxy. Mix up an ounce of epoxy, then mix in enough micro balloons to cool whip consistency.. that’s what you compare to the bondo. A gallon of epoxy would probably make 5 or 10 gallons of filler mixed with micro. The weight difference is huge! Micro doesn’t weigh anything, sands super easy and doesn’t shrink. Why would anybody use polyester bondo on anything except maybe a quick disposable mold is beyond me..
 
You can’t compare a gallon of bondo to a gallon on epoxy. Mix up an ounce of epoxy, then mix in enough micro balloons to cool whip consistency.. that’s what you compare to the bondo. A gallon of epoxy would probably make 5 or 10 gallons of filler mixed with micro. The weight difference is huge! Micro doesn’t weigh anything, sands super easy and doesn’t shrink. Why would anybody use polyester bondo on anything except maybe a quick disposable mold is beyond me..

same reason body shops use it, takes 15 min to be workable.

I use epoxy/micro (or flox) for the big fixes, then neat epoxy for pin holes; however best case is 12 hours to fully cure. I have some of the quick feather fill stuff for pin holes I find after primer.
 
I have had a very positive experience using Evercoat which I have used it for the near to final stages. Using Micro balloons is great for bigger gaps or waviness but not as easy for final stages.
 
Because it works

You can’t compare a gallon of bondo to a gallon on epoxy. Mix up an ounce of epoxy, then mix in enough micro balloons to cool whip consistency.. that’s what you compare to the bondo. A gallon of epoxy would probably make 5 or 10 gallons of filler mixed with micro. The weight difference is huge! Micro doesn’t weigh anything, sands super easy and doesn’t shrink. Why would anybody use polyester bondo on anything except maybe a quick disposable mold is beyond me..

We are not talking about much in the scheme of an RV. I used less than a pint of Evercoat 416 on my 7. The bulk of which was sanded off. For fine fill, sanding and time you can't beat 416. It is hard but sands easily. On my first 7, I used it and is probably knocking on 2000 hours now with no sign of cracks in any of the tail fairings where most of it was used.

Judicious use of proper materials is not a problem.
 
my rule is I don't mix chemistries. if your working on an old kit that has parts made from polyester, use polyester and micro. if you have the new epoxy parts, use epoxy and micro. simple enough. I getting ready to send my RV-4 to paint, all the parts are polyester, I use polyester resin and micro.

bob burns
RV-4 N82RB
 
I’ve got 1 dent on the skin of my elevator that I would like to fill as I blend the fiberglass tip once it’s installed.

Thanks for the help

Not sure I would use very much filler on a control surface to fill a dent. If you do, make sure you recheck the balance.
 
Bondo should go

After using Bondo on car repairs for many a year then finding epoxy and fillers for building my airplanes I would never go back to Bondo.
You will find out for your self. The epoxy and fillers sands much easier and becomes level faster. You can make something flatterer and blend in easier with the epoxy. Oh ya they attach better and usually with no shrinkage.
Oh by the way yes mixing is a little different and harder BUT it produces a much better working product.
You will get used to it and leave the Bondo to gluing shovel handles and the such back on
Way lighter to. Art
 
I thought the OP was talking about filling a dent. To make weight a consideration doesn’t seem worth the worry. The difference in weight is probably less than the weight of the bug that just impaled itself on that same part of the airplane. He didn’t say he was filling 100 dents, so weight is not a player here. If you are fairing your wheel pants or making a deep contour on your windscreen, I agree, epoxy is the clear winner. But for filling a dent? A high quality polyester filler designed for this purpose has worked millions of times, with good results. They both work, so take your pick. Nothing wrong with either for dent repair.
 
Polyester-based filler vs epoxy-based filler is not really a binary choice, all or nothing, one or the other. Both have their uses, and the reasons can be subtle.

Big picture: Epoxy is superior in terms of adhesion, tensile strength, and modulus. Polyester is fast, simple, and inexpensive, with (in very rough terms) 20~40 % lower mechanical values.

Considering only these mechanical factors, an epoxy filler becomes a better choice when the application will be subject to more stress. An example might be the leading edge of a surface subject to insect and rock impact, or bridging a gap subject to differences in thermal expansion.

A faired surface, with no notable stress, doesn't require high mechanical values. A polyester filler allows speed; it becomes possible to notice a small imperfection, apply fill, sand, and paint in the same afternoon. Polyester resin is also less expensive, as its viscosity and cure are adjusted with inexpensive styrene monomer, often as much as 30%. Speed and price are huge drivers in the auto body industry, where the application doesn't require high mechanical factors.

One practical factor can be application position. A simple epoxy/micro mix is not thixotropic. It will sag and run to some greater or lesser degree when applied to a vertical surface. It may not matter (for example, if the surface will be subjected to a lot of contour sanding later), or it may be quite inconvenient. All the pre-packaged commercial polyesters are largely thixotropic, and even it they were not, they cure so fast that it doesn't matter.

There are chemical factors. For example, polyester resin cannot be applied directly to foam, so it doesn't work for contouring fast forms in the home shop.

There's an old rule which says epoxy can be applied to a cured polyester/glass composite, but polyester should not be applied to a cured epoxy/glass composite. The polyester filler industry has pushed hard to develop fillers sold as "sticks to anything", so the rule may not be quite as ironclad as it once was. However, given the average aircraft builder has epoxy on hand in the shop, observing the rule has little downside.

Polyester shrinks during cure and later, which introduces internal stress, so eventual cracking was expected with earlier polyester fillers. We don't see so much of that with today's improved formulations, but shrinkage still exists.

Neither epoxy or polyester is particularly safe. MEKP catalyst is blinding. Epoxies are notorious for sensitizing users to the point of bad skin reactions. You must use care with either system.

Polyester fillers are heavier, as the typical bulk solids are talc and limestone. The primary bulk filler is most epoxy formulations is microballoons. It obviously makes no real difference to airframe weight if the quantities are small. However, be aware that a heavier filler does affect control surface balance. There is at least one flutter accident on record in which a significant quantity of filler moved a rudder CG aft.

To the OP's question...Polyester is probably fine on the dent. Epoxy/micro would be better for the seam between the fiber glass tip and the aluminum elevator. You could use either for either, and obviously some do with success.

Recent practical applications in my shop...Rans has builders cover bolt heads which protrude through the front of the tubular leading edge spar. The heads ultimately are under the fabric, thus the fill or a cover plate. Some filler also fairs the junction between the spar and the leading edge upper surface sheet metal. This is, hands down, a polyester filler job. Both wings, from start to done, took about 45 minutes.

Carbon turtledeck shell...one ply of 6K twill on each side of a honeycomb core. Although either polyester or epoxy filler would work, epoxy micro is much lighter in this quantity.
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