I just finished my first attempt at painting on Mylar / vacuum bagged wing with mixed results. I have bagged before (2 times) but this was the 1st attempt at paint transfer.
The paint turned out OK but my problem is with resin delaminated beneath the paint. What I mean by resin delaminated is there are voids between the cloth and the wing skin (cloth to paint ok).
The process I followed to build the wing is as follows:
1. Foam core sheeted with standard grade 1/16” balsa, light ply spar, carbon fiber tissue on TE for aileron support, bond via undiluted West System epoxy, vacuum bagged. 2. Cores sanded, routed for skinned hinge from .01” FG sheet top and bottom. Aileron torque rods installed. DHP Micro Filler fill voids around FG sheet and torque rod tunnels. 3. Wing sealed with West System epoxy (diluted with 25% of alcohol). 4. Mylar waxed, masked and painted with Kloss Kote paint and satin catalyst (over period of 2 days). 5. 18 hours after final colour sprayed on Mylar: epoxied strips of CF .5oz tissue to trailing edges top and bottom; epoxied .75oz X 2” wide FG cloth to LE of wing; Applied 3oz resin diluted with 25% alcohol on .5 - .75oz cloth directly onto Mylar. 6. Placed in Vacuum bag, 5-7” vacuum for 18 hours, room temperature. 7. Stripped Mylar; findings – Paint OK, some flaws on bottom due to reuse of Mylar; resin delaminating primarily around area of FG sheeting (skinned hinge).
Questions: I have had the delaminating before when I had not sealed the balsa, I thought sealing with thinned epoxy would solve the problem. It seems I may be trying to be too stingy on the use of resin, however the wings still seem heavy (I do not have scales to measure accurately) current weight probably 18-20oz without tips installed? How do others deal with sealing of balsa?
Should I apply resin to the wing as well as the Mylar?
Since I have a void beneath the FG Sheet (for later cutout and building of skinned hinge). Is this void area drawing out resin when under vacuum?
My thoughts for solving the problem is follow the same process as above with the following changes, please advise if you can suggest other solutions.
1. Bond contest grade 1/16” balsa to foam core under vacuum with glue other than epoxy – Polyurethane seem to be a preferred choice from some messages I have read. 2. Use a different filler (epoxy and balloons), since the Modeling filler I use (DHP Micro-fill) does not allow sealing with Balsa rite. (Balsa rite softens filler). 3. Seal balsa with Balsa Rite rather than epoxy to reduce weight. 4. Apply un-diluted epoxy to cores in the location of FG sheeting
Posts: 252
Joined: 8/14/2002 From: Shakopee,
MN, USA Status: offline
I have done over 50 wing this way and I would guess the thinning of the Epoxy that you use on the Mylar is the problem. I have sealed and not sealed the balsa and could not tell the difference. I do not seal the balsa now. Also MGS Epoxy is much superior to West. Sealing the balsa is adding 1 to 2 oz. per wing. Balsa skins should weight 2 oz. each, wing should weight 16-17 oz. with wing tips.
Thanks for your response, I have read some of your previous arcticles and this has provided great guidance to this point.
This is the 3rd wing I have vacuum bagged and the first time I have tried to thin the epoxy or seal the balsa. The reason I thinned the epoxy and sealed the balsa was in attempt to prevent this reoccurring problem with delaminated or dry resin sections. It appears that many, almost all of the areas affected in this wing are around the skin hinge.
I have routed the .01" FG sheet into the top skin of balsa .01" deep and epoxied this in place, then routed out the wing 5/16" wide, then routed in .01" thick FG sheet on the bottom, to be cut for the eventual hinge seal. Is there a chance that the void in the wing under vacuum is drawing epoxy away when finally being bagged? Do you do the hinge routing before you bag or after?
Posts: 1555
Joined: 1/6/2002 From: Team Coldies,
CA, USA Status: offline
That kinda looks like the FG you inlayed had bubbles in it already before you bagged the wing. My reason for saying that is because there are bubbles in the FG thats over the aileron groove.
A possible fix here for those bubbles would be to drill some tiny holes and flood out those bubble with some orderless CA,, then sand.
BV
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Posts: 896
Joined: 4/24/2002 From: Arvada,
CO, USA Status: offline
Kevin,
I got bubbles like that on my first couple of tries. I was using thinned Z-poxy finishing resin for the outer layer. The bubbles quit happening when I did 2 things simultaneously:
1. Switched from Z-Poxy to my regular laminating resin. I use Fiber Glast, most others use West Systems, but I think the principle is the same. Finishing resins aren't very strong, and thinning them reduces the adhesion even more.
2. Applied resin to the balsa as well as the glasscloth.
I think what's happening is the bare balsa is sucking the resin out of the cloth. Fill the grain of the balsa with something beforehand and that won't happen. You might use Balsarite or nitrate dope, but I just scraped a little bit of resin onto the balsa with a credit card. It doesn't take much.
Posts: 252
Joined: 8/14/2002 From: Shakopee,
MN, USA Status: offline
From photo it looks like the Mylar is not vacuuming down to the fiber glass. What mill Mylar do you use? Do you sand the balsa down to the fiberglass so there is no ridge? Also the vacuum bag should go over the Mylar then laid in the foam husks and wighted, vacuum pulled. I all ways seem to get one or two spots, dime to quarter size, that looks like gum ball, little ruff area.
I am using .014" Mylar. I took considerable time to ensure the FG sheeting was routed and laid flush with the balsa, and filled all of the voids / imperfections from that process with Micro-Filler. I then painted over this with epoxy. Once dried this was all sanded smooth.
I places the wing in the vacuum bag then back onto to the bottom husk and clamped the TE with aluminum angle and flat bar. However I did not weigh down the wing between the foam husks.
I am going to start over again, this time with particular focus on the FG sheeting, the rest of the wing really looks not bad for the 1st painted attempt (if I do say so myself). I thought I had paid close attention to the skinned hinge and a good fit of the hinge , at the same time I cannot help but think routing out the hinge gap afterwards would help to stiffen the area and lessen the chance for resin loss to this void (if this is happening).
Again thanks for the comments I am sure some of these successes / difficulties are based on personal techniques often subtle but with varying consequences. As they say practice makes perfect.
By the way has anyone tried to make the wing skin (painted and hinged) before it is glued to the foam cores. It would be easier for production of several skins or laminates for any kind of wing if the subsequent joining to the cores could be mastered?
Posts: 2837
Joined: 12/29/2001 From: Brantford, ON, CANADA Status: offline
When I first started vacuum bagging wings I used the mylar method. I always ended up with cavities under the cloth. I tried thicker and thinner resins, all quality products. It seems that as the mylar is pressed down by the bag an air bubble can form and has nowhere to go.
I do not use mylar anymore. I now use peel ply and breather cloth, problem solved.
I have not done another wing since the one I was discussing here. My wings are turning out to be too heavy. I need to fcus on 3 areas;
1. Use contest grade balsa. 2. I will substitute Polyurethane glue for the epoxy I was using between the foam and balsa. 3. I have been using CF mat (.5oz) on the TE for stiffness. The results have been excellent, very stiff and you could cut yourself on the edge, however there is substantial weight buildup to get the results. I think I have over done it.
In the mean time I am cheating and ordered 2 composites from Harold. I like the Seeker kit just need to work on the wing weight issue. I have studied the delamination I am getting and it is all around the skinned hinge, my current theory is the air / vacuum in the cavity is contributing to the issue.
Al are you and Kevin racing this year? Club, District or both?
Ed; on your note about peel ply. I have some of this material, never used, but I assume you are not painting until after pulling from the bag?
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Problems with delaminated resin in Vac bagged wing? - 
