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Old 06-12-2005, 09:57 AM
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laminating- gel coat pre-release

I've been doing repairs lately on a body that I laminated last summer. Unfortunately, I had several spots of varying size where the gel coat pre-released from the mold. Most were in concave areas where the slight shrinkage of the gel coat during curing appeared to cause the pre-release. It was hot weather when I did this gel coat application, which probably contributed to the problem, and I also suspect that I probably got higher film thickness of gel coat than I was targeting in these areas. By the way, this was the first laminate from a "green" mold, and PVA was used over a wax mold release.

Repairing these areas after the fact is tedious enough that I want to make sure in future work that I don't get into pre-release of the gel coat ever again!!!!! I realize now that I should have cut out these areas of gel coat before I laminated over them, but I didn't at the time. A learning experience.

So, here are my questions for the laminating experts on here:

1. Is pre-release a rare phenomenon on a structure as large as a body shell having many concave surfaces?

2. When doing an entire body shell, is it feasible or recommended to do the body shell in multiple sections as a way to minimize the length of time that a portion of gel coat is uncovered before receiving the first laminate? My thinking here is that by getting a first layer of laminate on the gel coat more rapidly, the probability of pre-release problems should be reduced.

3. In a car that is going to be painted anyway, would eliminating the gel coat altogether and laminating directly onto the mold be a viable option? I realize that a good gel coat job offers some improved appearance attributes...but I'd consider almost any option that would avoid pre-release problems in the future.

4. A second, unrelated problem that I had was voids between the gel coat and first layer of laminate, always in areas of tight curves such as around the windshield and door jambs. I feel fairly certain that I was getting the glass layed down well initially, but it was tending to spring-back before it cured. I was using 1.5 oz mat. Would a thinner mat, such as 1.0 oz, on the first layer of laminate tend to give less problems with spring-back and thereby reduce the number of voids over curved surfaces?

Thanks for your comments.

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Old 06-12-2005, 12:07 PM
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I will try a few suggestions

Quote:
Originally Posted by powderbill
I've been doing repairs lately on a body that I laminated last summer. Unfortunately, I had several spots of varying size where the gel coat pre-released from the mold. Most were in concave areas where the slight shrinkage of the gel coat during curing appeared to cause the pre-release. It was hot weather when I did this gel coat application, which probably contributed to the problem,

Probably might be due to using a hot mix of catalyst..In hot weather the cure is real fast so the cat needs to be cut back so the glass does not "go off so quick"...

Repairing these areas after the fact is tedious enough that I want to make sure in future work that I don't get into pre-release of the gel coat ever again!!!!! I realize now that I should have cut out these areas of gel coat before I laminated over them, but I didn't at the time. A learning experience.

Yup shrink back in tight areas is a problem..

So, here are my questions for the laminating experts on here:

1. Is pre-release a rare phenomenon on a structure as large as a body shell having many concave surfaces?

I think what you are calling pre-release is a shrinking of the laminate during cure which is pulling the gel coat up off of the mold..Slowing the cure down a bit should help..

2. When doing an entire body shell, is it feasible or recommended to do the body shell in multiple sections as a way to minimize the length of time that a portion of gel coat is uncovered before receiving the first laminate? My thinking here is that by getting a first layer of laminate on the gel coat more rapidly, the probability of pre-release problems should be reduced.

Getting well organized will help with this as the layup can go very quickly when you "get the rythum" so to speak...

Get Very well organized having a clean well organized workspace is vital to getting a good layup..Write down a layup schedule..with specs for the thickness of gel coat..precut all of your glass fabric..Stack your fabric so the first piece to go in is on top on down to the last piece to go in the mold is on the bottom of the pile..then you can just grab them and go..

3. In a car that is going to be painted anyway, would eliminating the gel coat altogether and laminating directly onto the mold be a viable option? I realize that a good gel coat job offers some improved appearance attributes...but I'd consider almost any option that would avoid pre-release problems in the future.

Gel coat is considered to be the final finish on a lot of glass work..carries the color and provides UV protection to the glass which glass does need..You can if you wish eliminate the gel coat and paint the glass..I would use a good epoxy primer tho in that case..Gotta be real good on your layup technique to avoid pinholes in what we call "see throughs" pinholes can be a customer satisfaction deal..

4. A second, unrelated problem that I had was voids between the gel coat and first layer of laminate, always in areas of tight curves such as around the windshield and door jambs. I feel fairly certain that I was getting the glass layed down well initially, but it was tending to spring-back before it cured. I was using 1.5 oz mat. Would a thinner mat, such as 1.0 oz, on the first layer of laminate tend to give less problems with spring-back and thereby reduce the number of voids over curved surfaces?

Rule of thumb is that the heavier the fabric the harder it is to "lay down" so going to a thinner fabric will help with this....Some times in very hard bends we need to make a paste with "Cab-O-Sil" or "Bondo to help make the bend..fill the corners with that material and use something like a tounge depressor with a radius end to lay the fill in and then laminate over that...That may help with those...

Thanks for your comments.
Takes some tinkering to "get it right" I worked in a boat factory and we were constantly adjusting to weather conditions to get the cat/resin ratio to work well..

And the workspace was very well organized and we had plenty of mixing cans..brushes and tool cleaning materials ready at hand..

All the fabric was precut according to the layup schedule so there was no stopping once layup commenced..Now saying that we could not get an entire 30 footer laid up in one shift so we had "stopping spots"

Pleas consider laying in some balsa core or foam to reinforce any large flat areas in your design..the contributes alot to making the glass structurally stable and long lasting..

Good luck

OMT
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Old 06-12-2005, 01:25 PM
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Thanks for the helpful comments, OMT. Your comments about fast gel times are well founded in this case...I (thought that I) wanted to buy the laminating resins in 5 gal pails rather than drums, and the only one stocked in pails locally was a super-fast gel time resin that was obviously not optimized for hand lay-up. So, I was faced with mixing very small quantities over and over, and was working all by myself without a helper. (In retrospect, I should have bought a full drum of the right material rather than fighting the fast gel just for the convenience of being able to store 5's in the cellar.) Just out of curiosity, when laying up a boat, how many people would you usually have working on the laminating? I can see where it would be quite a help to efficient workflow to at least have someone help clean tools and mix fresh batches of resin.

Also, in a typical 8 hour shift, would you spray the entire mold with gel coat at once even if you couldn't get it all laminated, or would you only spray what you could laminate in a shift? For that matter, did you spray an entire shift worth of gel coat once, or did you spray gel coat multiple times each shift?

Thanks again for your comments! Hopefully, someday I can get some pics posted on this site.
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Old 06-12-2005, 02:25 PM
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shift schedule

The entire mold was shot with gel-coat and then the first and second layer was done in one shift..maybe even the 3rd or 4th..depending on how fast we were working that day..No dawdling around allowed on layup day...Depends on the size as well in that area..the little dinghies were a half day deal to get it all done..

Usually 2 guys in the boat and one helper handing stuff in..these things were large enough that one coudl be standing in the keel arear and you could not see out over the edge..

The helper needs to be well trained so he can hand you stuff as you need it and he should be able to see what you need before you do..Kinda like an operating nurse to a surgeon sort of thing..
We did use chopper guns to run the mat and to wet out the fabric before rolling it out..

Lot of times the hull was laminated totally in one shift..next day we were installing the stringers and bulkheads...

Yeah mixing the correct cat/resin ratio is vital..I like a slower cure so I have time to get the glass down before the resin "goes off"..You may need to find another source for materials..

Something say the size of a 32 ford body is a one shift deal for a good laminator..Next day install all the reinforcements and such..let all that cure overnite and then you can pull the mold..won't hurt a bit to let it cure even more like a couple of days to be sure of the cure..Sometimes the boats were left in the mold quite a while until we had all the structure in before we pulled as the mold supports the shape until all the bulkheads and stringers are in..

Works the same for a car part..car parts are just considerably smaller...

I cannot stress the organization part too much..Maybe it is my own deal as I can get considerable "grumpy" if things interfere with progress when I am trying "make show" and get something done...

thanks

OMT
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Old 06-12-2005, 06:28 PM
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omt's advice is right on, the gelcoat prereleasing from the mold is from too fast of a cure rate. in the summer slow the cure down.

your supplier for your resins should have many different types of polyester putty available so you can make tight bends. 1oz. mat is pretty light stuff and might help a little but really your 1.5 oz. should be fine. if it wont make the bend then the 1oz will most likely have a tough time too. i dont think i would use bondo for filler though, unless you can spread it perfectly. you want a putty that has a slow cure so when you lay the mat down its still wet and the mat will push into it a little otherwise if your putty has a bump then you will get a small air pocket where the bump is when you lay the mat over it. i prefer milfiber putty myself. does the same job as what omt said but it has strength.

do yourself a favor and keep the gelcoat. you can avoid airpockets in fiberglass but there will always be very tiny pinholes in the glass when you pop it from the mold. these will make for a nightmare if painting. some people spray polyester primer into the mold too instead of gel. this way your part comes out primed and ready to go. the primer or gelcoat should be cured before layup of the part otherwise the resin can attack the gel and wrinkle it up. you wont see it until you pop your part from the mold.
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Old 06-12-2005, 06:59 PM
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This sounds like good advice, many thanks to you both.
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