If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
I have worked on several gliders over the past 25 years where the =
gelcoat cracks did indeed propagate down into the glass structure. One = in particular was an ASW-15 where we had to zip off the entire outer = layer of unidirectional glass in order to remove all of the cracks. = These were crazing cracks over the entire surface, not stress cracks = from load (crashing,etc.). A severely cracked LS-4 comes to mind as = well. With good light and a jeweler's loop, we have noted a good = handful of other ships that had localized crazing that had gone down = into the structure. Speaking of down under, the Australians have noted = this problem and issued an "AD" (or whatever they call it) concerning = this. Paul |
#22
|
|||
|
|||
One of the main reasons our gelcoat gliders crack up is because of the =
final finishing process. Boats are popped out of a shiny mold and are = basically done. Glider pieces that are gelcoated get additional sanding = for contouring, etc.. This breaks up the surface gelcoat which, at the = time of process, has additional "protective" cross-linking at the = surface. This is great for contouring, but a no-no for finish = longevity...unless you immediately paint over this surface with poly = paint, etc.. I have noted several gliders that were refinished with poly paint where = all of the old crazing was not removed. One $10K, 2-seater refinish job = has cracks returning after 3 years. Paul=20 |
#23
|
|||
|
|||
Ian,
I have had excellent results with a 'partial' gelcoat removal/refinish ... repainted with Simtek Prestec ... fuse 5 years old, still perfiect ... as are wings 3 and 2 years since refinish. This is a lot of work, BTW. Documented on SRA site ... http://sailplane-racing.org/Articles...asw20_wing.htm BTW ... did I say this is a lot of work. JJ ... apologize for stealing your punch line. :-) KK Ken Kochanski "Ian Forbes" wrote in message news Time for a round up. I started this thread 4 days ago and I asked: There has been quite a bit on refinishing on RAS recently. Most people advocate that when gel-coat starts cracking up, it is essential to remove it ALL. There has been lots of advice and theory as to why the above is true. No doubt it is the recommended approach - recommended by many who are not in a position where they might have to pay for the work! However there was little response to my question: Does anybody out their own a ship which was refinished without removing all the gel-coat? What is the service history of these gliders? So far there have been two postings of problems with re-finishes. Neither of these involved a polyurathane finish. Gliders re-finished with extensive or complete removal of the old gel-coat have held up - at least as well as the original gel coat. So I will extend the question. Does anybody anybody know of gliders that have been refinished with polyurathane and subsequently experienced problems? I can answer that by saying I know of one glider that was re-finished with polyurathane about 10 years ago. There is some localised paint cracking on the tailplane and rudder which suggests that there were some areas of poor preparation during the refinish. I suspect that the glass skin on these areas is thiner more flexible which results in a finish that is more susceptible to cracking. This was not an "expensive" refinish job, for example the ailerons and flaps were not removed during the respray. But overall the glider still looks very neat and I think I can say this finish has lasted as least as well as many factory gel coat finishes. Ian |
#24
|
|||
|
|||
Ian - painting a control surface without removing it means it has not
been checked for proper mass-balance, and for many gliders is likely way out of spec - flutter hazard and not airworthy. If you refinish over cracked gelcoat, the cracks will continue to propagate upwards through the finish and downwards into the laminate - not airworthy and hidden. A reputable and knowledgable mechanic will sign off neither of these "shortcuts". Be careful out there, Happy Holidays, Dave PS: Yes, I have seen gliders where these shortcuts were taken, and yes, it caused serious problems later. Ian Forbes wrote in message ... Time for a round up. I started this thread 4 days ago and I asked: There has been quite a bit on refinishing on RAS recently. Most people advocate that when gel-coat starts cracking up, it is essential to remove it ALL. There has been lots of advice and theory as to why the above is true. No doubt it is the recommended approach - recommended by many who are not in a position where they might have to pay for the work! However there was little response to my question: Does anybody out their own a ship which was refinished without removing all the gel-coat? What is the service history of these gliders? So far there have been two postings of problems with re-finishes. Neither of these involved a polyurathane finish. Gliders re-finished with extensive or complete removal of the old gel-coat have held up - at least as well as the original gel coat. So I will extend the question. Does anybody anybody know of gliders that have been refinished with polyurathane and subsequently experienced problems? I can answer that by saying I know of one glider that was re-finished with polyurathane about 10 years ago. There is some localised paint cracking on the tailplane and rudder which suggests that there were some areas of poor preparation during the refinish. I suspect that the glass skin on these areas is thiner more flexible which results in a finish that is more susceptible to cracking. This was not an "expensive" refinish job, for example the ailerons and flaps were not removed during the respray. But overall the glider still looks very neat and I think I can say this finish has lasted as least as well as many factory gel coat finishes. Ian |
#25
|
|||
|
|||
Paul,
I agree that this additional surface work could lead to, down the road problems. I do not think this is because of changes in the crosslinking, at least not entirely. Crosslinking is the bridging of the loose ended polymer chains. By designing the polymer of choice (in this case gelcoat) to undergo crosslinking you create one giant supermolecule. And during the painting process as long as each additional coat is applied before the crosslinking is finished, each additional coat will add to the matrix. But after the fact this chemical reaction will not take place to the same extent and a mechanical bond becomes the order of the day, hence the surface roughing requirement during repairs. *As a side note: the amount of crosslinking involved plays a large part in the amount of flexibility a finish will have. By using a flex additive you can inhibit the crosslinking in some or all of the paint. My company's work with interfacial chemistry has long shown however, that like a lot of polymers, even gelcoat is semi-permeable to water. So, for example when you wash your glider, then dry it and finally wax it (because it is all clean now, right?), your drying was never really able to extract the water from the pores. The wax seals it in and capillary action along with other hydroscopic forces pull (or drive) the water in deeper where it freezes and expands, pushing against the sanding scratches which now become stress risers, ultimately leading to microcracks which propagate to larger cracks. Without the wax the water is free to go, to and from, as external and internal forces change usually in relationship to the seasons or prevailing weather conditions. Reuben Paul Gaines wrote in message ... One of the main reasons our gelcoat gliders crack up is because of the = final finishing process. Boats are popped out of a shiny mold and are = basically done. Glider pieces that are gelcoated get additional sanding = for contouring, etc.. This breaks up the surface gelcoat which, at the = time of process, has additional "protective" cross-linking at the = surface. This is great for contouring, but a no-no for finish = longevity...unless you immediately paint over this surface with poly = paint, etc.. I have noted several gliders that were refinished with poly paint where = all of the old crazing was not removed. One $10K, 2-seater refinish job = has cracks returning after 3 years. Paul=20 |
#26
|
|||
|
|||
Bob, why do sailplanes (which spend most of their time in the trailer)
have such a problem, while sailboats (which spend most of their time in the sun and weather) usually have a good gelcoat? B Lacovara wrote: The composites industry has many years of experience and good understanding of gel coat cracking mechanisms. To address a few of the comments: 1) A gel coat fracture must be removed to the extent of its depth; otherwise the crack propagation mechanism is in play and will eventually affect the surface again. 2) A thick and/or flexible secondary coating will delay the re-appearance of cracks, but not the effects. There must always be concern about an underlying (but now covered) crack propagating into the laminate. 3) UV penetration of the laminate is a non-issue unless the crack is huge. In that case the crack itself will be a structural issue. 4) To date, there is not a viable method of "filling" cracks. This is a function of the inherent surface tension of the potential liquid materials. If one could fill cracks there is a fair chance two micro-cracks would appear on each side of the original crack and eventually propagate into larger cracks. 5) To date, no one in the composites industry has come up with a workable shortcut. A quick fix solution would be worth big bucks, with gliders being a tiny portion of the overall application. Hope this helps. Bob Lacovara |
#27
|
|||
|
|||
Reuben-
Fascinating post, would not waxing increase UV damage? |
#28
|
|||
|
|||
Oh My, Ruben
It sounds like we have finally found someone who actually knows what they are talking about. Your post brings up several questions: for example when you wash your glider, then dry it and finally wax it (because it is all clean now, right?), your drying was never really able to extract the water from the pores. Should we be waxing gelcoat? How about urethane finishes? I have seen little blisters that form when a sailplans is left in a very wet environment, like a metal trailer. When these blisters are popped, a little bit of water is evident under each blister. I have seen this in urethane and also in gelcoat (Prestec) What is your take on this? JJ Sinclair |
#29
|
|||
|
|||
At 16:00 21 December 2003, Jj Sinclair wrote:
Oh My, Ruben It sounds like we have finally found someone who actually knows what they are talking about. Your post brings up several questions: for example when you wash your glider, then dry it and finally wax it (because it is all clean now, right?), your drying was never really able to extract the water from the pores. Should we be waxing gelcoat? How about urethane finishes? I have seen little blisters that form when a sailplans is left in a very wet environment, like a metal trailer. When these blisters are popped, a little bit of water is evident under each blister. I have seen this in urethane and also in gelcoat (Prestec) What is your take on this? JJ Sinclair Ruben's post is really interesting and he makes a very strong case for water exerting a damaging shear force between phsically bonded coating layers a refinished glider and for exacerbating cracking - but is it the whole story for gliders with their original finish? I think Bob Lacovara also knows what he is talking about - see his posting of 04-42 on 6th December and also his article (and qualifications) at: http://www.wingsandwheels.com/Weathe...ock%20Tech.htm I will take a lot of convincing not to cover my glider with a UV protectant wax and, preferably, to have a PU finish to reduce the risk of surface crazing from UV degredation. Having suffered badly from water filled blisters on one glider I am very conscious of the effects of moisture but it seems, from what is written about experiences in the boating world, moisture will get in through the composite structure to the gelcoat from the inside out in wet conditions. The boating people say that the water dissolves chemicals out of the resin on its way through and that the water in the blisters tastes chemical but when I tried it on water from our Discus fuselage blisters it was pretty tasteless. For crazing I think that UV surface damage top old gelcoat is the main enemy and water can then start to penetrate from the outside. Meantime, in damp conditions, water has been penetrating from inside from new and this can cause blisters even in gliders with less porous coatings So my plan is to get a PU finish, use Wx Seal/Block during really dry spells of weather to keep the external surface as impervious to water and UV as possible and to store the glider in as uv-free dry conditions as I can achieve. John Galloway |
#30
|
|||
|
|||
I would like to beat this dead horse a little more.
I will check and rephrase my "cross-linking" term. I would like B. = Lacovara to explain the additional protection afforded at the surface of = a very shiny, smooth de-molded gel-coat part that is not aggressively = sanded/polished. I do know that in many cases this cracking/crazing = follows the exact pattern of the sanding strokes/motions that are = performed at the factories. Note the cord-wise cracking on LS products. = They used to do much of the sanding in that direction. They also used = jitter bug type sanders on the back bones of the fuselages. You can = observe these "eyelash" shaped cracks all down the surface of the spines = of many LS gliders. Schleicher's wing control surfaces crack = span-wise...the direction of sanding. Final grit numbers and direction = of sanding and follow-up polishing can make a huge difference in surface = longevity too. This was explained to me an a very large repair facility = in Germany a number of years ago when they demonstrated span-wise final = sanding in two stages, followed by a chord-wise direction of the polish = machine. DG employs this technique. Lacovara's paper confirms that = surface "roughness" allows faster degradation on several levels. You = can increase the life of your factory gel-coat finish by sanding the = surface with 1500-2000 grit paper and polishing ACROSS this sanded = pattern, increasing the gloss. There are a number of polishes suitable = for this, including the hard stick polishes used by the factories and = 3M's Finesse it and Perfect it systems, just to name a couple. Follow = this up with WX seal/block system, and you are well on your way to a = happier gel-coat surface. =20 Dry out your ship, slick it up, use WX block system once, preferably = twice a year, NEVER tie it out, and store it in your trailer inside a = basement or hanger, etc.. P. Gaines=20 |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Commercial Glider refinishing book | Martin Carolan | Soaring | 0 | December 15th 03 09:41 AM |
REFINISHING | Ventus B | Soaring | 32 | December 3rd 03 10:14 PM |
Older glass glider refinishing question | Gus Rasch | Soaring | 6 | November 10th 03 12:18 AM |
Refinishing Your Aircraft (now in Oregon) | aerocomposites | Home Built | 0 | September 24th 03 05:29 AM |
Tracon II Shortcut key for "THEN" | Jack Frost | Simulators | 0 | August 17th 03 02:54 PM |