Gelcoat softening when glassed

Discussion in 'Materials' started by idkfa, Jan 14, 2008.

  1. idkfa
    Joined: Sep 2005
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    idkfa Senior Member

    Hi all,

    I'm on the mould learning curve, spayed gelcoat into a mould and then laid 3/4oz CSM. I needed to reposition it a bit and on picking it up, found the surface of the gelcoat had gone soft (not all the way to the mould), it looked uncatalysed.

    1) How is it possible for polyester resin to affect a previous layer of cured resin like this? Being that gelcoat is polyester based.

    2) If so, then does this happen between layers of f/g?

    3) Can I make my own gelcoat by adding pigment and micro fibers/high-density filler to resin? Will this not be "harder” than gelcoat, or may be to hard? More importantly, not go soft?
  2. ondarvr
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    ondarvr Senior Member

    That's normal. The cure of polyester resin and gel coat is inhibited by air, so the surface will stay tacky even though it's cured below the surface. As the styrene in the resin attacks this uncured surface it does break it down some. It's the same for resin, you don't see it with resin because there's no pigment, this is what gives you a good chemical bond between layers.

    You could make your own gel coat, sort of, but you don't have access to most of the ingredients, so it would be a lesser product than what you could buy.
  3. Jimbo1490
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    Jimbo1490 Senior Member

    Exposure to oxygen prevents the surface from curing. This actually helps during laminating as subsequent layers will adhere to and cure that uncured film. To make polyester resin cure completely, simply cover it, either with a plastic film or even a thin layer of wax, as is the case with 'tack-free' formulas.. Anything to keep oxygen away during cure.

  4. idkfa
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    idkfa Senior Member

    Thanks guys for the reply. So, styrene has equal effect on cured gelcoat and resin?

    It seems to me that gelcoat use is problematic, alligatoring etc. print-through. Why use it in the first place? Why is it a better surface than pigmented, thickened resin? Cause it is harder?
  5. ondarvr
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    ondarvr Senior Member

    You didn't mention the other issues in the first post. Gel coat is basically thickend and pigmented resin, but there's a little more to than that.

    Alligatoring is mainly due to one thing, under cure, the under cure can have many different causes, but it's from the lack of adequate cure to resist the styrene from attacking it to the point of lifting and deforming.

    Thin gel coat, or not waiting long enough to apply the skin coat are the most common causes, but it can be from uneven cure due to mold shape, poorly adjusted equipment, poor technique, not mixing the catalyst into the gel coat well enough and others.

    On print through, some can be blamed on the gel coat, but the resin is the main culprit, a 20 mil layer of gel coat has very little strength and won't stop a 1/8 to 1/4 inch laminate from shrinking and distorting the surface. There are barrier coats that will help with print, they just move the laminate further away from the gel coat resulting in less print.

    Picking the correct resin for the job and getting a good cure while the gel coat and resin are still on the mold will help a great deal with print.
  6. AroMarine
    Joined: Jul 2005
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    AroMarine Junior Member

    Gelcoat is a bit problematic for newcomers sometimes even for those of us who have been making parts for years. When it works properly it is a great finish material for boats. Durable, repairable, polishable. I am not sure about your description of uncured alligatoring(many small ripples) or prerelease (soft flexible and loose from the mold) uncured (soft to the touch able to make a finger nail imprint. So I will shoot for alligatoring. Most of the time this stems from uncured gel caused by spotty catalysation. I am IInot sure of your temp there in UK but here it is pretty cool now. Gelcoat needs to be warm especially when catalysing. For small parts in the cooler temps I warm up the gel while I am mixing it using a heat gun and mix thouroghly if hand mixing a minimum of 2 mins. Gel is more visquous than resin so resin usually mixes better and in between layers the glass resin mix will keep uncured resin from wrinkling .
  7. fiberglass jack
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    fiberglass jack Senior Member

    thats normal, the gel is designed to remain tacky even after cure this way the resin used to skin will bond, for polyester resin to fully cure it must be blocked from the air, some resins will contain a wax which allows a full cure, if you use this resin for the skin layer u will have to sand the fiberglass for the second layer to bond, this is the reson that gelcoat for mould production does not contain wax, its normal for the gelcoat to get soft after you wet the mat this is the chemical bond, if the gelcoat is thin in spots you may get alagation which is the gelcoat softens and wrinkles , this can also happen if the resin takes to long to cure as the styrine in the resin is working overtime on the gelcoat, a trick that i do is after the gel is cured is to mix a batch of resin the same color as the gel make it hot and coat the gel with a thin layer when this is cured it wont soften as easy as the gelcoat and acts like a barrier coat to help stop print through,

  8. tinhorn
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    tinhorn Senior Member

    One weird cause of alligatoring I ran into was styrene gas settling in the bottom of a mold with high sides, which retarded the gelcoat cure. Tipping the mold so the vapors could "run out" solved that problem.

    I've made several parts with no gelcoat - just resin against a mold - and found they always had pinholes on the surface. And print-through is resin shrinkage, not poor gelcoat. Could you use a resin filler, increase your glass ratio, or otherwise remove more resin from the first layup? I used a thin ceramic/resin mix behind the gelcoat back in the day. (Your skin coat should be almost dry - not a drop of extra resin.)
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