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Old 09-13-2006, 09:29 AM   #1
prestige6
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Default 1958 Corvette Gel Coat Or Filler Primer??

Need advise from you experts out there. I'm getting mixed opionions on preparing my vette for paint. Some say Gell Coat while others say with todays filler primers is good. What say you all???
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Old 09-13-2006, 10:51 AM   #2
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Do repairs, seal, then spray polyester, block sand, prime, sand again, seal again, paint. On any new glass or repair we allow time for shrinkage either after the spray polyester step or the primer step. Put the car out in the sun or bake it under infrared to accelerate shrinkage.

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Old 09-13-2006, 12:58 PM   #3
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You are like hitting a Hornets nest. It depends on the amount of repair work done and if any aftermarket glass was used. Some aftermarket glass is Gelcoated already, some is not. If it wasn't from the factory, they I would suggest using Ecklers Spray Gelcoat due to the porosity of it. I use it and I like it. Then I use a Polyester filler primer for my next coat and let is sit in the sun or IR lights before using the paint manufacturer's recommended primer.
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Old 09-13-2006, 09:24 PM   #4
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The following information was supplied by a major manufacturer of fiberglass parts.
.
Building Barriers on Fiberglass
THE IMPORTANCE OF GELCOATING FIBERGLASS
Research and practical application continue to demonstrate the importance of applying gelcoat on bare polyester reproduction fiberglass parts as well as OEM fiberglass. These reproduction parts are manufactured from only fiberglass and resin as were the original parts which came on the cars and the subsequent replacement parts sold by GM. The original parts were not gelcoated. Our parts are also not gelcoated.
Not only is it important to apply gel coat on these new reproduction parts as well as original fiberglass; but it is also important to apply a minimum build of 20 mils. A sufficient barrier does not occur with lesser film build. (This is the same amount of gelcoat we put on a handlaid gelcoated part.) Achieving 20 mils of film build requires the application of approximately 4 heavy coats from a large orifice HVLP spraygun. These guns are now common in the automotive paint industry and used for spraying the new heavy build primers.
The most common resistance to spraying gelcoat is unfamiliarity with the product itself. Gel coat is no harder to apply than heavy primers and is an excellent base material for any paint job to be applied over it. Another reason for not using gelcoat is "the insurance company won't pay for it."
This is incorrect. If a repair shop shows the insurance company the documentation that the parts need to be gelcoated, the insurance company will pay for the extra procedures.
Bare fiberglass panels contain porosity. So it follows that creating a barrier on the surface of the part is very important. (It is also important that no liquids are used on the bare glass during the repair process as they will absorb into the laminate.)
The reason for creating a barrier: Bare fiberglass is so absorbent that it will collect liquids and vapors that come in contact with it. If no barrier exists over the fiberglass, virtually any liquid or vapor (including humidity) can migrate through the pores in the laminate from the backside of the part to the outside of the part causing many small pockets of liquid to form in the pores directly under the primer and paint surface. As enough vapors collect to form liquid in the pores of the laminate, hydraulic pressure is formed.
This hydraulic pressure creates enough force to lift the primer and paint off the surface. This lifting will be visible in the form of blisters. (The liquid under these blisters is normally water mixed with whatever the water has pulled from the laminate.) The correct thickness of gelcoat will create a barrier between the fiberglass and the paint, preventing these blisters from forming.
Anyone who has worked on old Corvettes extensively has seen this blistering phenomenon on cars that have come in to have an old paint job stripped off. This is how paint jobs deteriorate on a fiberglass car. This is part of the normal aging process of a paint job on a Corvette. In the sixties, we stripped and gelcoated cars by the time they were only a year old; because the paint was already blistering off. These cars were produced in an era when paint jobs were expected to last 3 years or less.
Testing has shown that gelcoat thickness is also very important, because it cannot be an effective barrier if the gelcoat is too thin. The most dramatic test to date is a panel sprayed with varying thickness of gel coat on the same panel. After exposure to severe conditions, the area with the lesser amount of gelcoat shows blistering; while the area with the correct thickness showed no blistering.
To repeat: Bare fiberglass reproduction pressmolded (bare glass to match original) and original panels require a barrier to prevent the migration of liquids and vapors as described in this article. The importance of this barrier cannot be stressed enough. These polyester fiberglass panels are best sealed with polyester gelcoat.
This information is being presented to help you make the best possible choices when refinishing an early Corvette constructed of it's original polyester panels or the press molded bare glass panels made today. It is, however, impossible to account for repair techniques or conditions in shops all over the country. Shop conditions, repair and refinishing techniques all have bearing on the longevity of a paint job; and there is no parts manufacturer who can guarantee long term results.
Please DO NOT, I repeat, DO NOT use urethane and epoxy primers on reproduction bare glass parts. The pores and fibers in the laminate will absorb the heavy solvents in those products and will migrate out later to cause blisters.
.
TECHNICAL BULLETIN
(ISOTHALIC) POLYESTER GEL COAT
CATALYST MEKP CLEAR LIQUID
CATALYZATION RATE: 10-15 CC 's PER QUART
STRAIN GEL COAT INTO GUN . MIX CATALYST WHEN READY TO SPRAY.
GEL COAT WILL SET IN TEN -FIFTEEN MINUTES , SO IT IS IMPORTANT TO
APPLY QUICKLY BEFORE IT SETS IN GUN IF YOU STOP SPRAYING , CLEAN
YOUR GUN WITH ACETONE OR LACQUER THINNER
COATS: YOU MAY SAFELY APPLY MULTIPLE COATS WITHOUT WORRYING ABOUT EXCESSIVE BUILDUP. DOUBLE COATING LOW AREAS TO REMOVE WAVES AND RIPPLES IS ACCEPTABLE. ( IT IS IMPORTANT TO ACHIEVE 20 MILS FILM THICKNESS OF GEL COAT. IF YOU SAND GELCOAT OFF DURING BLOCK SANDING IT IS IMPORTANT TO APPLY MORE GEL COAT TO MAINTAIN FILM THICKNESS .) THE PREFERRED METHOD IS TO SIMPLY GELCOAT A SECOND TIME. SEE GELCOAT WET TO CURED CORRELATION TABLE BELOW FOR ACTUAL CURED FILM BUILD
NOTE: WET FILM BUILD CAN BE MEASURED WITH A FILM THICKNESS GAUGE AVAILABLE FROM GEL COAT SUPPLIER.
FLASH TIME: THE FRESHLY SPRAYED GEL COAT SURFACE WILL START TO LOSE GLOSS INDICATING THAT IT IS TIME TO APPLY ADDITIONAL MATERIAL. IF YOU ARE SPRAYING A WHOLE CAR ,YOU SHOULD BE ABLE TO KEEP SPRAYING UNTIL YOU ARE ALL DONE ; BECAUSE THE MATERIAL FLASHES FAST ENOUGH TO ALLOW YOU TO CONTINUE GOING AROUND THE CAR
CURING: REMEMBER -GEL COAT WILL NOT CURE PROPERLY IF NOT SEALED FROM THE AIR
AFTER YOU HAVE SPRAYED YOUR LAST COAT OF GEL COAT, APPLY PVA (POLY VINYL ALCOHOL). PVA IS USUALLY GREEN OR BLUE IN COLOR SO YOU CAN SEE THAT YOU HAVE APPLIED ON ALL SURFACES. PVA SEALS GEL COAT FROM THE AIR ,SO THE CHEMICAL REACTION TAKES PLACE IN THE ABSENCE OF AIR. PVA RINSES OUT OF YOUR GUN WITH WATER.
PVA MAY BE WASHED OFF WITH WATER AFTER 48 HOURS OR SOONER IF YOU CAN BAKE IN A HEATED SPRAY BOOTH AT 140 DEGREES OR MORE.
GELCOAT WET TO CURED CORRELATION
MILS WET......... MILS CURED
.....24..................... 16
.....28..................... 19
.....32..................... 24
SANDING: AFTER WASHING OFF PVA, GEL COAT IS TO BE SANDED DRY ONLY. AFTER SANDING GEL COAT DRY ,BLOW OFF SURFACE .
DO NOT USE ANY LIQUIDS ON THE SANDED GEL COAT BECAUSE THEY WILL BE ABSORBED. CAUSING PAINT PROBLEMS AFTER YOU HAVE BLOWN THE SURFACE CLEAN, YOU MAY APPLY OTHER PRIMERS FOR FINAL FILL AND BLOCK SANDING. (I REPEAT: IF YOU SAND GELCOAT OFF DURING BLOCK SANDING, IT IS IMPORTANT TO APPLY MORE GELCOAT TO THESE AREAS. )
WARNING: DO NOT APPLY POLYESTER GEL COAT ON SHEET MOLDED COMPOUND. IT IS NOT COMPATABLE WITH SMC AND WILL NOT PROPERLY ADHERE.

Last edited by buns; 09-13-2006 at 09:34 PM.
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Old 09-13-2006, 11:15 PM   #5
John McGraw
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That is exactly correct. Gelcoat and polyester primer/surfacer are different products, which serve different purposes. I use Gelcoat to seal the glass and old repairs, and then I use primer surfacer to give build to block sand and to provide a topcoat primer. There are lots of guys that skip the Gelcoat, but I will never do so on my cars! The primer/surfacers contain way too much inert material to be an effective sealer like the pure polyester resin that is in Gelcoat. The inert filler material makes the primer very porus.

Regards, John McGraw
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Old 09-14-2006, 01:17 AM   #6
Russ T Gate
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In 1990 I took my 67 to bare glass and did all necessary repair work, and then used gelcoat to seal all errant strands. my paint and body work has lasted for 16 years with no bleed through or bubbling of the paint. It is now time for a repaint and I will touch up all repair work with a gelcoat seal and finish with a sanding primer. Hopefully this will last for another 20 years.
Good luck with your decision
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Old 09-14-2006, 10:41 AM   #7
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Quote:
Originally Posted by John McGraw
That is exactly correct. Gelcoat and polyester primer/surfacer are different products, which serve different purposes. I use Gelcoat to seal the glass and old repairs, and then I use primer surfacer to give build to block sand and to provide a topcoat primer. There are lots of guys that skip the Gelcoat, but I will never do so on my cars! The primer/surfacers contain way too much inert material to be an effective sealer like the pure polyester resin that is in Gelcoat. The inert filler material makes the primer very porus.

Regards, John McGraw
Amen!
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Old 09-15-2006, 12:45 AM   #8
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Quote:
Originally Posted by unreel1967
Amen!
I 2nd the Amen! or at least until NASA gives up their secret squirrel formula, if it exist.

Craig
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Old 09-15-2006, 02:16 AM   #9
AZDoug
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OK, but lets get to the bottom of this.

SOMETHING, that looked like sprayed on gelcoat has been applied on top of the fiberglas, and underneath the paint on two virgin cars that I chemically stripped back in 1974.

What was this SOMETHING? it does not remove with stripper, it is grey color, and it is several thousanthds of an inch thick, and it looks like sprayed and sanded gelcoat.

Did the factory spray on gelcoat on the assembled body, and work the gelcoat using the usual methods of sanding/blocking before prime and paint?

Doug
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Old 09-15-2006, 01:47 PM   #10
John McGraw
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Doug,

The factory only used lacquer primer/surfacer on top of the bare glass.
I have never seen anything but the dark red/brown primer used on C1 cars even light colored cars. This primer does not strip too well because it is porus and sucks up stripper like a sponge. I usually strip all the paint and then spray stripper on the primer, and scrub it off with a coarse scrubbing pad.
The factory never used any catalyzed coating back in those days, just lacquer. Even the spot putty used to fill pin holes in the bonding adhesive was a thick putty that was lacquer based, very similar to the nitro-stan putty used to this day in many body shops.


Regards, John McGraw
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Old 09-15-2006, 04:44 PM   #11
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Quote:
Originally Posted by John McGraw

The factory only used lacquer primer/surfacer on top of the bare glass.
I have never seen anything but the dark red/brown primer used on C1 cars even light colored cars.
Ok, thanks.

if this is the case, then what I was told was *original paint*, may have been a dealership repaint, that included the application of a grey gelcoat, or grey something under the primer. I had no reason to doubt the gelcoat was factory, as it would seem odd that someone would strip an entire car down to bare fiberglas to apply a gelcoat, back in the early 1960's on a damage or warranty repaint.

I remember the red primer also, it did chemically strip, you just had to use more elbow grease with the plastic Bondo squeegee to get the primer off.

The only thing I can say is it would seem a lot of people put gelcoat on repaints back then, based on the half dozen or so that had it that I worked on. I did see some without the gelcoat, also, just primer.

The cars were painted prior to installation of the windshield assembly, correct? As i recall there wasn't gelcoat under the paint on the dash board when I replaced the dash pad which would indicate the gelcoat was done ex-factory.

Doug
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Old 09-15-2006, 05:16 PM   #12
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Quote:
Originally Posted by buns
Bare fiberglass is so absorbent that it will collect liquids and vapors that come in contact with it. If no barrier exists over the fiberglass, virtually any liquid or vapor (including humidity) can migrate through the pores in the laminate from the backside of the part to the outside of the part causing many small pockets of liquid to form in the pores directly under the primer and paint surface. As enough vapors collect to form liquid in the pores of the laminate, hydraulic pressure is formed.
This hydraulic pressure creates enough force to lift the primer and paint off the surface. This lifting will be visible in the form of blisters.
I have a few of these blisters on my car. I will now strip it completely with a razor blade, fix the stress cracks, Gelcoat it, then prime/block the car.

This is great info, thanks
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Old 09-16-2006, 05:10 PM   #13
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Quote:
Originally Posted by AZDoug
The cars were painted prior to installation of the windshield assembly, correct? As i recall there wasn't gelcoat under the paint on the dash board when I replaced the dash pad which would indicate the gelcoat was done ex-factory.

Doug
Yup, the windshield was installed well after the car was painted.
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Old 09-16-2006, 07:09 PM   #14
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John McGraw... Do you Gelcoat the undersides of your bodies also, or do you strictly use the tintable undercoating?
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Old 09-16-2006, 11:42 PM   #15
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On my restorations, I try to leave the underside alone. If it is real bad and needs some work, I will tint some gelcoat to match the color of the original glass. The matched-die press-molding process that GM used to produce these parts, yeilded a very dense part that had a resin-rich layer on both sides of the part that is similar to a gelcoated surface. If the surface is untouched, the need for gelcoat is minimal, and on the underside, it can cost you points on a restoration. Putting gelcoat under other coating on the topside of the body, is a must in my book, even if the body is undamaged. I would rather be safe than sorry.

On a hot-rod, I usually spray the underside with bed liner material that is tinted to the desired color, whether it is to match the paint or black.

The main reason that the aftermarket fiberglass people are so strong on reccomending gelcoat, is that even theough they call their parts press-moulded, the process has little in common with the original process used to produce the parts. It does not produce parts that are as dense, that have the resin-rich layer on both sdes, or are as well cured. The original dies were heated and cured the part very hard, very fast.

Regards, John McGraw
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Old 09-17-2006, 09:12 PM   #16
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John,

The red prime can be a bear to get off the car, but a little trick we used to use is plain old number three grade steel wool and paint reducer. It takes some work but takes it off pretty easy. Dip the steel wool in a bucket of reducer and then gently wipe the same area for a little while, (The reducer seems to soften the primer) then lay in to it with some elbow grease and it should come right off. Probably works as good as the stripper and the sanding block.. Just my 2 cents worth…

Ernie
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Old 09-17-2006, 09:12 PM
 
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