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Two honeycomb void questions...

Michael Burbidge

Well Known Member
Filling honeycomb voids

Preparing the cowl is taking me forever. The most difficult part is filling the honeycomb voids. See picture below. There's lots of information in the forums about pinholes, but I don't think the voids are pinholes. Is that correct?

How is the easiest way to fill these voids?

Thanks,
Michael-

The following pictures is after applying several coats of epoxy, sanding and applying an epoxy primer. But I still have voids in some places.

83E3EOK.jpg
 
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Somebody just posted a detailed method using drywall spackling. Search for that thread and you will be a happy camper.
 
The "voids" that you refer to in your picture will become pinholes once you spray on your primer. I followed Van's directions for filing the voids in the fiberglass with BONDO and shaving off the excess...it took several weeks to go over the entire cowling surface.
 
We used epoxy AWL fairing compound, it is light weight but pretty pricey
It is a two part epoxy can be sanded easily and does the job. With out pin holes. Put it on with light coats and it can go over the primer you already shot just roughen it up with some 320 paper and clean it.
I was amazed at the number of pin holes in the fiberglass parts. But handled them with a razor blade and some Dolphin glaze.
Available from Aviall and recommended by Sam James.
Smilin' Jack

Still Painting.. 3weeks so far.
 
We used epoxy AWL fairing compound, it is light weight but pretty pricey
It is a two part epoxy can be sanded easily and does the job. With out pin holes. Put it on with light coats and it can go over the primer you already shot just roughen it up with some 320 paper and clean it.
I was amazed at the number of pin holes in the fiberglass parts. But handled them with a razor blade and some Dolphin glaze.
Available from Aviall and recommended by Sam James.
Smilin' Jack

The AWL is a good way to go, but I think I'd scuff the primer off with 100X and then use the epoxy fairing compound. In general, it's best to not put filler on top of primer. Your cowl looks a lot better than some of the glass parts I've helped RV-4 guys with!
 
Preparing the cowl is taking me forever. The most difficult part is filling the honeycomb voids. See picture below. There's lots of information in the forums about pinholes, but I don't think the voids are pinholes. Is that correct? How is the easiest way to fill these voids?

Correct.

The traditional filler would be glass microballoons in mixed epoxy, aka "micro". You would squeegee a thin coat onto the bare glass prior to any other application, cure, sand flush, seal with a few unthinned epoxy coats, sand again, then shoot primer.

BTW, AwlFair LW 8200/7200 is fundamentally the same thing, but premixed and 10x more expensive. Take a look at the MSDS and tech sheet...bisphenol A epoxy, glass and phenolic bulk fill. Same is true of PolyFiber's SuperFil.

The following pictures is after applying several coats of epoxy, sanding and applying an epoxy primer. But I still have voids in some places.

I'm guessing you thinned the epoxy? Several coats of of unthinned West should have built up enough to fill the honeycomb depressions, even without micro. Still, it does look like you got it sealed, no pinholes, at least as well as I can tell in a photo.

The question is what to do now. You need a process which will work over the epoxy primer. Sanding to remove the primer won't really work, as you would still have primer down in the depressions.

First, your epoxy primer probably has a recoat window. Don't let it expire. If necessary shoot another coat within the previous coat's window.

PPG, SW, etc, all have high-build primer/surfacers. They are generally talc (the filler) in an acrylic binder. Primer/surfacers are intended to be sprayed over fresh epoxy primer. The idea is to spray on a heavy coat, cure, then block sand to remove all the filler from the high places, leaving it to fill the lows. It will take a few rounds, but this is work that a good painter would do anyway, and it can be sanded for tooth and painted later without restriction.

In PPG, look for K36 or K38.
 
Followup question...

Thanks all for the responses.

So Dan on the other half of the cowl, I will start with micro-balloons. I've used that in other places to do buildup. Then I assume after the voids are filled and smoothed, a few applications of epoxy, followed by an epoxy primer.

What consistency is best for filling these voids?

Thanks,
Michael-
 
So Dan on the other half of the cowl, I will start with micro-balloons. I've used that in other places to do buildup. Then I assume after the voids are filled and smoothed, a few applications of epoxy, followed by an epoxy primer.

You'll sand off all of the micro, leaving only what remains down in the voids.

What consistency is best for filling these voids?

Whatever works. I'd suggest peanut butter.
 
Use UPO-670

U-POL 670 Liquid Gold Pourable Glazing Putty. Google it!! I purchased mine through Amazon.com. Or get it direct from TCP Global. It is a two part epoxy - sands extremely easy after only 20 or 30 minutes dry time. Super great for filling these pinholes.. Van's method of acetone and epoxy resin is fine for the INSIDE of the cowl, but not the outside. As someone mentioned it is too hard to sand.
 
U-POL 670 Liquid Gold Pourable Glazing Putty. Google it!! It is a two part epoxy - sands extremely easy after only 20 or 30 minutes dry time.

Jim, perhaps you should Google the difference between epoxy and polyester resin systems. U-POL 670 is just another cheap polyester product. Save it for the gel-coated polyester/glass wingtips, or your hot tub.
 
I've built 5 composite airplanes over the last 35 years, and helped numerous RV'ers with their glass parts. I wish there was a magic bullet for finishing glass, but the best advice I've seen coming on this board is coming from Mr Horton.

I can't tell you how many wonder products I've heard touted over the years, but if you want to see how the experts finish one-off composites go visit SCALED or the sailplane folks --- and you'll see lots of dry micro/epoxy and almost no polyester.
 
At the risk of serious rebuttale

There is really only one right way to do this and if you are at all worried about weight, you want to do this the right way. Use an epoxy filler, such as an Epoxy resin system (such as West, Aero-Poxy or Rhino) and micro-balloons if you are on a tight budget. If you are willing to spend a bit more $ to save a lot of :mad::mad::confused: then get a pre-mixed epoxy filler such as Super-Fil. This will make the consistency of the surfaces to be sanded equal between filler coats.

The Van's cowlings are epoxy resin composite parts, so polyester resins (Bondo) are not really appropriate, although they have been used for COSMETIC FILLING on many a project with good results, but they need to be really small voids.

The problem I see now is that this surface has already been primed. :( It needs to be sanded with 80 grit to remove the primer and then you need to try to get the primer out of the voids. Then you can start anew and fill with the epoxy filler. Fill, sand with 80 grit, then fill the spots you missed and sand with 80 then 120 grit, fill the pin holes and sand with 120 grit. Then use a sand-able high build primer. Prime, sand (120 to very thin), prime, sand (120 to very thin ), prime, sand (120, then 240). Now if you were building a composite plane, repeat for the entire exterior :eek::eek:.

If you have any questions, give me a call.
 
There is really only one right way to do this and if you are at all worried about weight, you want to do this the right way. Use an epoxy filler, such as an Epoxy resin system (such as West, Aero-Poxy or Rhino) and micro-balloons if you are on a tight budget. If you are willing to spend a bit more $ to save a lot of :mad::mad::confused: then get a pre-mixed epoxy filler such as Super-Fil. This will make the consistency of the surfaces to be sanded equal between filler coats.

The Van's cowlings are epoxy resin composite parts, so polyester resins (Bondo) are not really appropriate, although they have been used for COSMETIC FILLING on many a project with good results, but they need to be really small voids.

The problem I see now is that this surface has already been primed. :( It needs to be sanded with 80 grit to remove the primer and then you need to try to get the primer out of the voids. Then you can start anew and fill with the epoxy filler. Fill, sand with 80 grit, then fill the spots you missed and sand with 80 then 120 grit, fill the pin holes and sand with 120 grit. Then use a sand-able high build primer. Prime, sand (120 to very thin), prime, sand (120 to very thin ), prime, sand (120, then 240). Now if you were building a composite plane, repeat for the entire exterior :eek::eek:.

^^^^ Exactly right
 
The problem I see now is that this surface has already been primed. :( It needs to be sanded with 80 grit to remove the primer and then you need to try to get the primer out of the voids.

Might not be a good idea to 80 grit the honeycomb at this point. It would be easy to cut the skin plies too thin over the cell walls...the high spots between the voids.

Tim, Dick, you think the voids are too deep for a high build acrylic urethane like K38?

Mike, this is block sanding a high build on a glass canopy frame. Routine stuff. See the dark streak in the lower right corner? That's the dark epoxy primer peeking through. It means I don't want to sand any further in that area.

zmmtf6.jpg
 
Might not be a good idea to 80 grit the honeycomb at this point. It would be easy to cut the skin plies too thin over the cell walls...the high spots between the voids.

Tim, Dick, you think the voids are too deep for a high build acrylic urethane like K38?

Mike, this is block sanding a high build on a glass canopy frame. Routine stuff. See the dark streak in the lower right corner? That's the dark epoxy primer peeking through. It means I don't want to sand any further in that area.

zmmtf6.jpg

Dan, as you know ideal situations are rarely achievable in homebuilding. Since the cowl isn't structural and Van's pre-pregs are pretty tough I'd probably block sand the affected areas (if they're regional and not endemic); and then try to dig out some of the primer with a small wire wheel in the Dremel. If 10 minutes of that didn't achieve the desired result I'd go to plan B and use K38 or something like it. If the epoxy primer in the weave could be mechanically abraded without tearing up the glass I'd go the dry micro route and then a high build epoxy primer. If the small wire wheel tears up the pre-preg than I'd go right to the epoxy primer. The key to dry micro is to wipe pure epoxy on the glass surface, wipe it all off with a paper towel , and then put LOTS of dry micro on. 95% will end up on the garage floor but this method is the best and fastest way to get perfect contours. You don't want to have to make 2 passes with dry micro as the epoxy interface between the two passes is hard and makes contour sanding much more difficult.

All of this discussion on glass surfacing is EXACTLY why, after 5 plastic airplanes, I'm going to learn to rivet via the RV-14. I'll be the one posting dumb riveting questions but I'm not at all concerned with glass. I intend to build my own wing tips from a John Roncz design and will build cowlings from scratch and layup a set in carbon pre-preg.
 
Thanks all!

The problem I showed in the picture is regional. In fact that 5x5 area I showed is the worst. The voids barely show through in some other spots, but some high build primer should cover those. There are two areas, the one I showed and the same area on the other side of the cowl, but it is not as bad there.

The information here is great though. Thank you so much.

I don't mean to shut this thread down, keep responding if you see fit. It is educational for all! I will be using what I learned here to do a better job of filling the voids on the bottom cowl.

Michael-
 
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