Need help for best methods to fill an area (Fiberglass cowling)

[Todd Breda]

My builder did a nice job overall for my 82 Pak but left me with the job of smoothing out the sides of the cowling (see circled section in the photos). The 82 Pak had these channels/vents that protruded halfway down the sides of the cowling. The place where he added the extension pieces can clearly be seen and felt. I need to make the transition seamless and smooth.

I am assuming I need some kind of bondo to putty and sand that area? Appreciate any and all expertise!

 

[Todd Breda]

Quick and dirty Photoshop sample of what I am trying to do...

 

[Mike Cathey]

I prefer to use West Systems with the 205 quick hardener along with the 410 filler mixed until it's thick but still spreadable. Much more durable than bondo and still sands easy. A used utility knife blade makes a great precision spreader.

Just make sure you've given the cowling a good cleaning with warm water and mild detergent to remove all the parting wax and mold release agents and rinse it off good and dry it completely.

 

[FloridaScaleBoater]

I thought we had answered this question previously?
Bondo alone will continue to Crack. Bondo has no strength.
Glass the seam with light cloth & epoxy, inside and out.
Once cured, sand and feather seam.
Then fill imperfections with Bondo and feather sand again.
Epoxy prime sand & paint.

 

[Todd Breda]

Thank you guys, I appreciate the info! @FloridaScaleBoater your method sounds great but unfortunately that's assuming I know how the heck to do any of that! LOL...I'll have to see if I can find some vids on how to use that method. Thanks again fellas.

 

[Todd Breda]

Alright, I bought a list for the job. Here's what I have:

Denatured Alcohol
Paint Brushes
Fiberglass Cloth (Thin)
Mixing Cups and Sticks
Z-Poxy Finishing Resin

Please let me know if these steps sound about right. Again, you are dealing with a total newbie here.

Sand and clean the surface area.

Mix the resin with the denatured alcohol for a watery substance (saw it on YouTube).

Brush on the substance onto the area.

Lay the precut cloth down to the area.

Brush over the cloth with the substance.

Let dry, then lightly sand.

Repeat process if necessary for seamless finish.

Yay? Nay?

 

[Mike Hughes]

Close enough. Speed up the process and layout several layers of cloth at the same time, then sand to shape.
Mike

 

[dr gary turner]

I prefer to use West Systems with the 205 quick hardener along with the 410 filler mixed until it's thick but still spreadable. Much more durable than bondo and still sands easy. A used utility knife blade makes a great precision spreader.

Just make sure you've given the cowling a good cleaning with warm water and mild detergent to remove all the parting wax and mold release agents and rinse it off good and dry it completely.

For better strength I use milled fiberglass. When mixed with 2 part epoxy it is like having mini re- bar in the epoxy.

I get mine from Airplane Spruce. Very inexpensive.

 

[Steve White]

Use a Dremel and sanding drum to smooth out the transition first, then build it back with glass. Do dry layups. For that application, look at the layup material schedule in the part and replicate it. You want the part to be as close to contiguous as you can get it to maintain even flexing. If you build up an area that's more rigid, it will transfer stress unevenly and crack. That's why not to use filler only.

Grind smooth first, then apply the patch so it blends in as close as possible to the original material thickness as close to invisible as possible for best results.

 

[FloridaScaleBoater]

Alright, I bought a list for the job. Here's what I have:

Denatured Alcohol
Paint Brushes
Fiberglass Cloth (Thin)
Mixing Cups and Sticks
Z-Poxy Finishing Resin

Please let me know if these steps sound about right. Again, you are dealing with a total newbie here.

Sand and clean the surface area.

Mix the resin with the denatured alcohol for a watery substance (saw it on YouTube).

Brush on the substance onto the area.

Lay the precut cloth down to the area.

Brush over the cloth with the substance.

Let dry, then lightly sand.

Repeat process if necessary for seamless finish.

Yay? Nay?

I would not use the denatured alcohol to thin the epoxy. Some epoxies do not react well to it and hinders proper curing.
Sometimes it remains rubbery.
Just mix your epoxy properly and use it as is.
Cabosil mixed with yer epoxy does make a good finishing putty.

 

[Todd Breda]

Thank you all for the replies and info!

 

[Steve White]

I can't tell the OML (Outer Mold Line) relationships from the pictures to best advise. Consider removing material from the outside and building it up from the inside. Maybe a combination of both techniques - add material inside and outside. Use a Long Board to work it. You can make your own from a piece of wood or other fairly rigid material - maybe 1" wide and 6-8" long or use a commercially available block.

Don't be afraid to remove material, easy to build back up. Without going through it or too far into it to remove high spots. It may be necessary to build it up from the inside to remove high spots on the outside.

Again, I can't tell from pictures - bottom line is it's best to remove the high spots and build structural integrity from the inside, than to fill low spots on the outside - within reason of course.

When you start check the OML overall contour with a straight edge to determine best course of action. It could be just a nice big low spot to fill with no high spots to grind off.

 

[Phil Mitchell]

Alright, I bought a list for the job. Here's what I have:

Denatured Alcohol
Paint Brushes
Fiberglass Cloth (Thin)
Mixing Cups and Sticks
Z-Poxy Finishing Resin

Please let me know if these steps sound about right. Again, you are dealing with a total newbie here.

Sand and clean the surface area.

Mix the resin with the denatured alcohol for a watery substance (saw it on YouTube).

Brush on the substance onto the area.

Lay the precut cloth down to the area.

Brush over the cloth with the substance.

Let dry, then lightly sand.

Repeat process if necessary for seamless finish.

Yay? Nay?

You'll get a better result if you clean it before sanding.
Don't thin the epoxy. open the crack a bit and clean it again. Acetone works well as does fuel alcohol. cut your pieces to fabric and test fit them. brush on neat mixed resin and fit the first of your cloth pieces. the rest of the steps are good. Epoxy has the possibility of amine blushing. If it does, wash with soap and water and a scuff pad before proceeding.

As for a filler, epoxy and microbaloons is better than just bondo.

 

[Todd Breda]

I can't tell the OML (Outer Mold Line) relationships from the pictures to best advise.

Thanks for the info! Here's some more photos...It's not a major lip or bump, just enough to be noticeable if painted over which I don't want.

 

[FloridaScaleBoater]

We're making "rocket science" out of a simple seam repair.
Just put some epoxy/cloth/epoxy on it....
Hell, if you don't like way it looks, grind it out & try again!

 

[Dave Gilles]

finishing epoxy is not meant for strength, its made for laminating so I personally wouldnt use it. Bondo is easy to sand and smooth out. its made for exactly this. anything that will bond and build up will work. thickened regular epoxy. Not thinned, dont add acetone or alcohol. you want thick. take an angle grinder with scuffing pad and dig into the material a little, fill with epoxy/bondo. sand with a small sanding block. hit with spray can primer to check your work is smooth.

 

[Bill Gibson]

finishing epoxy is not meant for strength, its made for laminating so I personally wouldnt use it. Bondo is easy to sand and smooth out. its made for exactly this. anything that will bond and build up will work. thickened regular epoxy. Not thinned, dont add acetone or alcohol. you want thick. take an angle grinder with scuffing pad and dig into the material a little, fill with epoxy/bondo. sand with a small sanding block. hit with spray can primer to check your work is smooth.

Respectfully disagree Dave! The weakest laminating epoxy is still 5 times stronger than Bondo will ever be as the ingredients that make up bondo are polyester. Filling and shaping with epoxy products and glass takes a little more effort but results in a far better piece! I only use Bondo or Icing for super minor imperfections/pinholes! As Rick says use epoxy/glass/epoxy and if it looks bad just grind it out and do it again!

 

[Todd Breda]

We're making "rocket science" out of a simple seam repair.
Just put some epoxy/cloth/epoxy on it....
Hell, if you don't like way it looks, grind it out & try again!

Question, given the photos I submitted earlier in the thread, how much cloth should I put down? Less is more? Like if I cover the seam, should it expand 1/4" on both sides of the seam? 1/2"? 1" or more? Thanks!

 

[FloridaScaleBoater]

1/2" either side.
MOST will be sanded away

 

[Bill Schaare]

Trick I've learned and used in glassing large scale rc planes involves using 100% polyester dress lining (Hobby Lobby/ Jo Anne Fabrics) often referred to as Peel Ply. Add layer of dress lining wetted out over the last layer of glass cloth and let dry 24-48 hours. Overlap the dress lining 2-3" so you have something to grip when removing. When dry the polyester lining can be pulled up at a 45 degree and will remove any of the amine blushing, leaving a smooth surface needing very little sanding.