Fiberglass/CF & Epoxy Frustrations

[Knundrm]

Need some pointers from those of you more proficient/getting better outcomes with their work in this area.

I invariably either end up with dry spots (white airframe) when using a fiberglass sleeve and shrink wrap or (black airframe) when trying to vacuum bag with porous teflon and batting (to absorb excess epoxy).

I have yet to have a "perfect" finish with these processes. I do like the results but, in the end, I am spending countless hours reworking the airframes to get them comestically appealing. I had not intended to fill and paint the black airframe but will likely have to do so because sanding down to remove the holes/divets will invariably cut into the fiberglass.

I would appreciate any pointers from the form to get better results. At this rate, I'm going to have to take up strong drink to get me through all of this rework!!!

 

[watheyak]

Those look like trapped air bubbles that are causing the dry spots. I feel your pain!

I tried in vain for a long time to get good results vacuum bagging tubes. I came from R/C sailplanes and I had vacuum bagged many wings. They all came out of the bag with a pretty much perfect finish every time. So it made sense that I should be able to do that with a tube, right? Wrong...

Getting a perfect finish right out of a the bag is really difficult to do. I stopped vacuum bagging tubes a long time ago and never looked back.

I had to readjust my expectations and ideas about what vacuum bagging was even for. These days I feel like vacuum bagging is to compact the layup and and remove excess epoxy. I also think vacuum bagged or otherwise compressed layups are stronger and lighter than ones made without compression.

When it comes to finish, getting a glossy clear finish that shows off the material below is just plain elbow grease, unless you're using a mold. Lots of sanding and a good clear coat. Jim Jarvis' method explains it pretty well.

There are some ways to get closer, though. These days I use a spiral wrap of heat shrink tape for compression. Still requires some sanding, but not as much. It also takes some practice to get right.

In the case of your black tube it looks like you could brush on another layer of epoxy, sand it all smooth and either polish it or clear coat it.

 

[n27sb]

If used properly sleeves do a great job.
When you wet them out try striking the resin by using both hands from the middle out. Wrap your fingers around the mandrel and squeeze out the excess by pulling in opposite directions. This also tightens the sleeve.
After it cures sand the high spots. Add a thin coat of resin and sand. Repeat until smooth.

 

[JohnCoker]

I think the best cosmetic finishes I've seen come from using clear automotive paint rather than extra epoxy for the clear coat. I've used Mylar to make a smooth surface, but always end up with a dry spot or two as well. However, I always paint my rockets, so this isn't fatal. Best tips I can give are to keep stretching the cloth as you roll and remove as much epoxy as possible.

I use fiberglassing for reinforcement of the tube, not for cosmetics. If I wanted the carbon to show, I'd use a commercial tube since they also don't have a seam where the cloth ends.

 

[H_Rocket]

One trick I heard of (and have not tried) is to "bag" your layup with Monokote (or other heat shrinkable film. The results can be quite stunning. You essentially compress the layup and ad color at the same time.

 

[n27sb]

I think the best cosmetic finishes I've seen come from using clear automotive paint rather than extra epoxy for the clear coat

I agree John. PPG 2 part economy clear puts a nice gloss on things. But I still fill and sand with epoxy if you are going for that nice CF look.
Since CF is an overkill for 24mm I can afford to cut into the surface quite a bit. my typical tube thickeness runs between .012 and .015.
After finishing out a couple of airplanes from glass cloth to paint, rockets are easy.
BTW not me in photo, but the paint is original even after 30 years.

 

[stantonjtroy]

I do this for a living and see this more often than I'd like to admit. Two things to keep in mind.
The ideal resin/epoxy : glass ratio is between 40:60 & 30:70 by weight. Without an autoclave 30:70 is not realistic. That being said, a slightly resin/epoxy rich layup is ok for a hobby rocket. Try measuring the weight of each component before the layup to achieve the right ratio and go with a non-perforated Teflon. This will reduce (if not eliminate) the need to pull out "Excess" epoxy. Some of what I'm seeing looks like resin/epoxy starvation, probably from too much epoxy being pulled out. Could help.
The other thing to watch is the amount of vacuum being pulled. Many people go for the max pull available at their elevation (ex. 27-29inmerc at around sea level). More often than not, 12-18inmerc is plenty. A full on hard vac can cause the epoxy/resin to cold boil leaving air bubbles and minor voids. FWIW.

 

[SDramstad]

Wrap it with peal ply after it is wetted out with resin and use a squeege to work the excess epoxy and air bubbles out.

 

[Knundrm]

Thanks for the feedback and insights folks!