Covering over fiberglass tanks and Laker Leading edge

I'm getting ready to cover my wings and wanted to clarify how to properly cover over the fiberglass components. The manual says to apply 2 coats of polybrush over the fiberglass, then I assume cover as per the poly fiber video (glue just the edges of the wing (leading edge, trailing edge, inboard rib #1, outboard rib #10).

I have a concern whether the fabric will not adhere uniformly over the fiberglass components and have some areas with adhesion, and others without. Is this a valid concern and if so, how does one avoid this?

Also, what is the best way to ensure the covering adheres to the underside of the wing where it concaves in? It seems like heat will pull the covering away from the ribs in this concave area...

Thanks in advance!

Mike,

You'll never go wrong following the polyfiber manual....:)

The LLE was not available when I biuild out Kitfox, So I can't claim any first hand knowledge about that.

For the fuel tank top & bottom rib:

Preventing delamination and making sure stuff sticks is definetly technique related.

Gluing the fabric down just along the edges with poly tack should always be done in a way so there are no voids between the fabric and whatever you are gluing the fabric to - if a person puts on enough poly tac and immediately pull/presses the fabric into it you should see the poly tac come up through the pores of the fabric - that is a solid attachment - never let it dry out before sticking the fabric. If the poly tac just touches the surface of the fabric lightly - you stand a chance of a less than adequate bond. If a person gets lumps of poly tac due to a little too much PT- that is what the iron is for - you have to get a smooth surface right from the start or the lumps will show through everything else that goes on top - as you practice - you learn what the right amount of poly tac is. The best way to make sure the poly tac joints are good is to be sure the poly tac penetrates the fabric and leaves no voids underneath the fabric. I found a good poly tac joint on the concave side of the ribs will hold just fine against the shrink (I used 350 degrees F) - just be sure to slather on the poly tac over ever area of the ribs and press/pull the fabric into the poly tac - use enough poly tac - don't let it dry out - and press the fabric in solidly.

On the wing tank, I believe the way a person would go wrong is if the coats of poly brush applied to the wing tank top are NOT thick enough before the fabric is applied; and/or, following the shrink - if insufficient poly brush is used or inadequate thinning is used or the poly brush is not worked into the fabric well enough you could get some voids/bubbles underneath - you are trying, in part, to dissolve the poly brush applied to the tank top earlier and melt it into the new poly brush being applied to the fabric. Cool temps are your friend - that allows you to work the poly brush into the fabric well before it has a chance to set up and capture bubbles/voids underneath. I know this is kinda radical;) but I found if I followed the instructions precisely (poly fiber manual and the guidance in the build manual) the fabric will become one with the top of the tank. :)

FYI - I rib stitched the entire wing except for the rib under the center of the fuel tank - it's still stuck and stuck well.

The wing tank - absolutely no delamination anywhere.

I found some inexpensive "Military chemical gloves" at the local military surplus store - they are thin enough that a person has good tactility, almost elbow long - you can smear them up with poly tac, poly brush, aerothane - whatever, and wash them off with MEK - I used one pair for the entire project by cleaning them up with a MEK rag - the good part is that a person can use their fingers to press materials into the poly tac hysol, etc and the gloves do not come apart like the wimpy floor exam gloves or nitrile gloves; and, the MEK does not soak through at all.

Another trick I learned from a poly fiber rep was to use a hair permanent application bottle for the poly tac - you can squeeze out a nice ribbon and work rather quickly with the bottle. Unbelievable, but you can let the poly tac sit in the plastic bottle for 6 months and it won't hurt the bottle - (makes me wonder why anyone would subject themselves to a hair permanent if the stuff is so bad you have to have a container that holds up to MEK!)


Sincerely,

Dave S
KF 7 Trigear - flying
912ULS Warp Drive

Fabric adhesion to the tank is good as long as you follow the instructions as Dave summarized below. The only problem I had was a couple of spots on the top of the tank were dished in and I didn't notice it. When I shrunk to 350 and then applied the polybrush, those dished in spots wouldn't stick down. There are only two of them and they are not very big, so I left them as-is. In hindsight I should have leveled them out with Super Fill before covering.

It is not clear in Dave's summary, but don't glue (Polytak) the ribs to the fabric before shrinking, except the tip and root. I believe the manual says to put two coats of Polytak on the ribs and let it dry. Then cover and shrink. THEN reactivate the Polytak underneath with MEK diluted Polytak (I think. I am going by memory). This method allows the shrink process to equalize over the whole wing surface, rather that one bay at a time. On the bottom concave surface when you reactivate the Polytak you will have to press the fabric down and probably use some weights to hold it down while it dries. Later you rib stitch and it won't come loose.

Mike,
Your concerns were exactly the same as mine as I was finishing my first Model IV. I had seen a couple of airplanes done as the manual suggests. When the top fabric was then brushed with the Polybrush it bonded tightly with the Polybrush on the fuel tank top where the fiberglass and fabric touched and left very definite lines where it broke contact with the lower non contact surfaces. Final paint (Polytone) left a marble effect above the tanks. That time, I made up some spacers of thin aluminum strips that I could slide over the top of the tank from the open end of the wing when applying the Polybrush brush coat. The spacers held the fabric above the tank surface so there would be no adhesion. I did it sequentially in strips until all was coated with nothing adhering to the tank top.

Then some discussion on the lists suggested with no adhesion there and no rib mid span there might be some ballooning over the tank. Friends flying along side said no, but I still wondered.

With the second Model IV, I laid down several layers of masking tape along side where the rib would normally be leaving a one inch wide channel over the tank. I laid down some "Micro" - a light weight bondo like material - into the channel between the tapes. Then after curing and removing the tape, I made a sanding board that ran between ribs 1 and 3 so I could sand the micro to match the height of the ribs. The fabric was then glued to the ribs as usual and the fake rib as well. I did not paint the Polybrush on the top of the tank.

As far as the Laker Leading Edge, I doubt adhesion will be a problem. I did the leading edge thing but before the factory option became available. I used Aluminum and there was no problem doing as the manual suggests.

All this being said, I think an Aerothane finish pretty much masks the marbelling effect of the Polytone I saw in the old days.

Yeh, I did fake the rib lacing for consistency.

Per Jim Ott's comment...that's correct - I didn't fill in the details :o on poly tacking/shrinking the concave sections of the the lower rib caps in my original post.....about all I recall about that is the poly fibermanual covers the operation quite well.

Some folks on the list have rib stitched the lower rib cap under the tank - I think by looping the stitch through some holes drilled in the rib web below the tank.

Dave S

Lowell,

Your paint job really looks nice.....:)

Dave

Thanks Dave. A note on rib stitching below the wing tank. I made a needle of soft heavy gauge wire (coat hanger). By flatting the wire with a hammer I could then drill through the flat part with a fine drill. This enables bending the needle as needed. What I found was that I could stitch by passing over the cap strip to one hole further along than I wanted, then immediately passing through that same hole along side the lacing cord back to the desired hole.