I am in the process of refinishing a 30 year old fiberglass boat which has obvious gelcoat crazing. I have discovered a two-part polyurethane primer specifically designed to fill gelcoat cracks and will be applying this primer (without removing the damaged gelcoat) prior to my two-part polyurethane finish coat. Having just been involved with paying for a $16,000 refinish job on our club's ASK-21, I would like to test my procedures on a glider gelcoat for the benefit of all those owners out there who are observing these nasty little cracks. I am looking for a specimen part of a glider with gelcoat crazing ....does anyone have a discarded part that I can experiment with? This will be a long range program, as I intend to place this refinished specimen out of doors year-round for some time to test the longevity of the finish. I will, of course, be observing the condition of my refinished boat at the same time. My test is being supported by Interlux Yacht Finishes and will be followed with interest by several certified sailplane composite repair stations. I realize that the only 100% cure for gelcoat crazing is complete removal and replacement, but if this system lasts for a number of years, it may be a viable and less expensive solution. Please let me know if you can help; any glider part will suffice: aileron, empennage members or a part cut out of a damaged fuselage or wing.
Joe Volmar, Adrian Soaring Club (Michigan, USA)
I think that to establish the validity of your procedure for gliders you would have to include a flexing regime for the test component and not rely simply on exposure to the elements. Glider structures are subject to a lot more flexing than boats are.
Also the extra weight and of a primer and then 2 pack PU finish on a glider without first removing some gel coat would be a more sensitive matter than in a boat.
At 03:18 18 January 2002, Joe Volmar wrote:
Your comments are very valid. I had planned to sand off enough old gelcoat(as evenly as practicable) to compensate for the additional weight. I don't know yet how to attack the flexing test but will work out something depending upon the glider part available for the test. I also plan to subject the item to some extreme temperature changes during the summer to simulate high altitude conditions. I also should duplicate low air pressure but do not have the facilities, as this is more or less a 'low tech' experiment.
A quick comment would be to suggest that you really need to think about what scale of gelcoat cracking are you proposing to deal with. There is a huge difference between the onset of the problem with fine initial cracking as observed with a quality viewer, and full depth gel coat cracks which may have existed for some time and are blindingly obvious to the naked eye. Full depth gelcoat cracks can enter the underlying resin matrix and produce structural cracks. If you just sand back part way in the severe case and overpaint with some system albeit clever, how do you know what the story is a the resin/fibre interface with the failed gelcoat, ie that you don't have structrural cracking?
Nothing I have seen or heard about the gelcoat cracking issue with sailplanes makes me at all comfortable with the idea of overpainting bad gelcoat cracking. You could ruin a glider if you don't deal with bad gelcoat failure by thorough & complete removal and refinishing carried out competantly. On the other hand there maybe scope for a process which addresses the other end of the scale of problem, ie. initial gelcoat failure evident under magnifiacation but hardly visible to the eye except under critical lighting.
I'm curious, What causes you to believe that the gel coat crack can then continues into the resin/glass,or(carbon fiber)? I'm not flaming you, I wish to understand your belief. thanks!
It happened to my glider, a Pegasus. We had a mid wing mounted wing dolly as part of our one-man tow out kit. The wing flexed at twice its normal frequency at this point. The gel was badly crazed, we thought due to trips to high altitude early in its life. A friend re-gelled it for us at a very low price and found that the crack went down into the resin. Charged us at cost price for the glass and resin to repair it. Just painting over after a rough rub-down could have made life interesting for us after a few more years.
But did the cracks from flexing the wing when cold and move down into the resin, or up from the resin into the gel due to flexing around this point? Who cares, have a good look, don't paint over it.
John Wright, 742
What usualy happens is sunlight can penetrate the cracks and UV decays the resin matrix underneath it is usualy not structural.
It has been clearly established that full depth gel-coat cracks can lead to problems in the resin-fibreglass matrix underneath. Gliding Federation of Australia Airworthiness Notice 69 issue 3 of June 1987 records two of our Australian professional workshops in 1985-86 finding clear evidence of crack propagation from the bottom of the gel coat and through into the top skins of a Hornet wing, Mosquito wing and Cirrus 75 tailplane.
Samples from the Hornet and Std Cirrus tailplane plus another sample from a different Std Cirrus were analysed at the Dept of Civil Aviation Materials Evaluation Laboratory by a materials specialist, and I have the report in hand. (1986 Report X-5/86, 'Deterioration of Glass Fibre Reinforced Plastic Gliders' by A Romeyn.) Conclusions: Some deterioration of the glass fibre reinforced epoxy laminates underlying the cracked gel coat was found. The deterioration was limited to matrix microcracking and some localised swelling and void formation, there was no evidence of fibre breakage.
Most of the information I have collected is from the 80's decade. The Germans established a sub-group of their industry-academic Working Group for New Fibre Reinforced Plastics ('Arbeitkreis Neue Fasserverstarktekunstoffe' to look at the gel-coat cracking problem in 1987. It would be interesting to get ones hand on the final outcomes of the work they did on the issue and any more up to date info from the 1990's.
Roger Druce Australia