Silkspan...Again

It's been a while since the "Silkspan Problems" thread was current, and I would like to compare notes with this slight update. Perhaps some of you who were still using your supplies of the earlier stuff at that time have now tried some of the current products. Rather than restating all the earlier observations about the current Sig and K & S "silkspan", I'll just explain what I found with another product and see whether anyone else has had similar results.

Last night in completing repairs to the subterranean "SkyRay" (club member says, 'three more Skyrays, and you'll be an ace!'), I decided to try out the Brodak silkspan that they generously sent on a trial basis a few months ago. Wing covering patches range from tiny patches, through an entire bay, to an entire semispan piece behind the spar. Since this plane can now be re-covered without damage to appearance, it seems the ideal test vehicle for the new material; both workability and durability (and how!) can be tested.

As previously noted, Brodak's told me that they only market "medium" weight silkspan under their name, and sell Sig in light and heavy grades. Thus I was surprised to find that it measured .002" or less in thickness - the same thickness I have noted on other "light" grades ('medium' to 'heavy' grade - 'old stuff' as well as new - mic out at about .0035"-.004"). While whiter, it also has some of the physical characteristics of the current heavier brands (crackles when bent, a bit 'blotchy' rather than evenly fibrous, etc.), although it does seem to have a slightly discernable "grain" direction.

I applied as always by first saturating with water sprayed in a mist (over towel) and then stretching as tight as possible across the gap. Brodak dope, thinned about 50/50 was applied aroiund the edges, and the material was again pulled radially outward to remove any new wrinkles, a more difficult task due to the thinness of the material and need to cut it short of the wing edges. Although finger pressure was sufficient to remove all wrinkles and waves, the material was probably not quite as tight as when it can be pulled over the wings edges. I was also concerned about wet strength and didn't want to risk tearing it. When dry the paper was tight and smooth. The periphery was doped again to ensure permanent adhesion, with no changes when dry. So far so good.

The big difference came when applying clear to the new silkspan. It immediately wrinkled sharply with many small, sharply defined waves ('crinckly'). These remained after the surface was pretty dry to the touch, but disappeared over about an hour's time. After four coats, this continues to happen, but the effect is less pronounced.

This pretty standard technique applied to the older, now unavailable silkspan did not result in initial wrinkling. I haven't patched with the heavier new stuff, although as previously noted, it went pretty slack on the P-Force, especially on the brushed coats, before tightening satisfactorilly.

So what's significant to me here is the sharp wrinkling of initially taut material when the dope is first applied. I will be interested in its strength when finished too. Any other experiences out there?

HI Serge. Mine did the same thing, but I just kept adding 50/50 dope until it got nice and taut. In fact, it got almost to tight in one area!! So, if you do it right, it works just fine. Easier than Poly span as to compound shapes such as wign tips, easy to repair. Just use a light touch and trust the dope to get it tight as it dries.

Ty-

I'm curious and would just like to verify the Brodak thickness - i.e. make sure they didn't inadvertantly send light stuff instead of "medium" (maybe I'll call John). Does yours measure .002" thick?

I have put on another couple coats of 50/50, separated by several hours away from the house. The first (less thinned) coat crinkled up more that before; the last wrinkled a lot less. The covering was nice and tight/smooth before each further application. I am noticing that the texture is getting to be smoother than heavier weight material. If it holds enough dope to have good strength, the weight savings may still be significant. I don't presently have plans to do a really valid quantitative strength test, but plan to judge by practical field experience whether to use it on a serious contest model. (ASSuming it survives the next bouts of outside maneuvers - to be attempted with awareness of SSW Forum suggestions!). Thanks for your input.

Serge

Serge, mine miked out to .0025 and .003" Somehow I fully expected it to be at .005+/-.

Ron B.
F4Fguy

While it may be true,I wouldn't be too quick to blame the tissue.I've had the same thing happen with SILK!.In my case ,I finally traced it to the dope.Both Sig Lite Cote and Brodak clear came up all "crinkly" until they were fully cured,and the LiteCote caused the covering to sag badly.Sig Super Cote was much better but took far longer to gas off.Sig nitrate didn't crinkle or sag. Full scale Randolph had none of these problems,neither nitrate nor butyrate,even the low shrink materials.

>Ron B.
>F4Fguy
>
>While it may be true,I wouldn't be too quick to blame the
>tissue.I've had the same thing happen with SILK!.In my case
>,I finally traced it to the dope.Both Sig Lite Cote and
>Brodak clear came up all "crinkly" until they were fully
>cured,and the LiteCote caused the covering to sag badly.Sig
>Super Cote was much better but took far longer to gas
>off.Sig nitrate didn't crinkle or sag. Full scale Randolph
>had none of these problems,neither nitrate nor butyrate,even
>the low shrink materials.
>
>Ron B.

It is most likely the dope and how it reacts to the glue in the paper. Silkspan is only paper after all, and the glue which holds it all together is 90 percent of the whole. I have had 40 plus year old tissue which was some of the best (recently) I have ever seen or used, and some "fresh" stuff which I wouldn't use to wipe paint brushes clean with. It just "all depends" on who made it and how- (when and where are meaningless).

Ron-

What you say makes sense; however I can see Jake's point that it is probably a combination of dope and "silkspan" characteristics. The "new" stuff marketed as "silkspan" by the major suppliers seems to have smaller paper particles - not such noticeable strands - and more of whatever matrix holds it together. Hence the "blotchy" appearance and crinckly flexing characteristics. Unless Brodak changed their dope formula recently, the new paper is not as good. What I bought a little more than a year and a half ago went on well with Brodak clear, sagging only minimally - if at all - and adhearing well to the doped sheeting. It was soon "tight as a drum". The newer Sig and K & S silkspan exhibited the inferior qualities described in the earlier thread, perhaps partly because it has less surface area and ability to absorb (I have done no density tests).

That said, I can report that I have finished repairs, using the Brodak silkspan (.002" thick) for the first time. Wet rippling diminished but never ceased with successive coats, but it always dried taut. It is now quite tight, ringing with a high pitch when tapped. It did take less dope, and for my utilitarian purposes (no appearance points - just a practice plane), it finished with a smoother unsanded surface, uneven from brushing, but with no textural roughness. A few intermediary bad laps from the Fox really revealed the resonant frequency of the large patch though - quite audible! If it bites the dust again, I'll save some covering to test, although that might be an unfortunate test in itself. Right now, I'm just too far behind in everything to spend much time on it. So far so good on that front though: no crashes tonight!

FWIW, it does seem to have come out lighter than what was originally on the wing. Even with overlaps, a lot of CA where there previously were no joints, extra epoxy, and microballoon/epoxy root reinforcement fillets, in putting the pieces back together and with fuel and oil remnants, it weighs now only about 1/2 oz more than it did dry before the crash.

Ty-

I forgot to ask whether your thickness was measured with fresh or doped Brodak silkspan.

Thanks for your input, guys.

Old (presumably "better") silkspan is an easy, cheap score at a swap meet. You see it all the time, usually for sale by RCers cleaning out their shop. Currell

Hi Serge. It was miked before any doping.

I've had this happen a *lot* on my test runs using silk. However, I got the technique sorted out with a lot of help and direction from this board, and the covering on the model worked out great.