Building From Mold without Gelcoat

Discussion in 'Fiberglass and Composite Boat Building' started by weldandglass, Apr 10, 2017.

  1. weldandglass
    Joined: May 2010
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    weldandglass Junior Member

    I'm in the process of finishing up a mold for an 18ft skiff of my own design. I'm adding plywood stiffeners to the mold and about to de-mold from the plug and see how everything turned out.

    I'll hopefully start building the first hull shortly.

    For several reasons, I'd like to not use gelcoat as an in-mold coating and post-paint the hull and cap using a quality 2-part like awlgrip.

    If I use duratec gray surfacing primer in place of gelcoat (it sounds like this should work but please chime in on relevant experience) and I'm backing the cured duratec with an epoxy laminate, my thought is that print through won't be much of an issue but I'm looking for guidance from anyone who's been down this road.

    I'm hoping I can get away with one layer of 3/4 oz mat behind the duratec then start with the biaxial-epoxy laminate.

    Any input is grealty appreciated.
     
  2. ondarvr
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    ondarvr Senior Member

    Their VE primer is a better option for below the waterline use if it's in the water for extended periods of time.
     
  3. groper
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    groper Senior Member

    I've used an epoxy high build primer as an IMC. It works so I reckon you'll be gtg :) put a good thick layer on and a light veil cloth and print shouldn't be an issue.

    The problem I had came later... Some of the wax etc from the mold ends up on your primer and when you sand it and prep it for final paint it's easy to end up with contamination on the surface prior to applying your awl grip final coat. Pay special attention to solvent cleaning and removal all traces of mold release after you pop your hull before you start sanding for final paint.
     
  4. weldandglass
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    weldandglass Junior Member

    Groper,
    thank you very much. I had given some thought to high build epoxy primer but have seen a lot of people going the duratec route so my brain was moving in that direction. I'll do a test run with the high build epoxy primer.

    On a separate but related note, I've gone over a lot of your posts regarding infusion and you were a great help to Jorge on his project. I've got quite a bit of wet-bagging experience but haven't done infusion yet. I'm hoping to solicit your input when I start.

    Thank you very again for the input.
     
  5. rxcomposite
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    rxcomposite Senior Member

    The purpose of the gel coat is to add color and provide an impervious outside layer. Resin without filler or solid contents work best. Apply a light coat of clear resin before the 3/4 oz mat. Without the first coat of resin, it would not be 100% perfect surface.

    Applying sanding primer in lieu of gelcoat is a bad idea as it is more porous than a gelcoat. Gel coat itself has a lot of solid content.
     
  6. groper
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    groper Senior Member

    You missed the idea RX- the finished hull is to be painted in 2 part polyurethane paint. Applying the swnding primer as a mold coating simply gives something to sand into as part of of the prep work for the final painting work. He doesnt want a gelcoat on the hull, thats why he asked the question...
     
  7. rxcomposite
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    rxcomposite Senior Member

    You misunderstood what I wrote Groper. Substituting a sanding primer in lieu of a gel coat is a bad idea. When the surface is in constant contact with water, a resin rich layer is used as an outer layer as a barrier. As I have stated, gelcoat is not the best as it has a lot of solid content. Pure resin is the best.

    In commercial applications such as water pipes and tanks, a resin rich layer of veil or surfacing mat is used in the initial layer (inside, where it contacts with water), followed by a light CSM.

    Sanding primer contains a lot of soft solid particles to make it easy to sand. In some cases, microbaloons is used. It is not a good water barrier. Images from Marine Composites.
     

    Attached Files:

  8. ondarvr
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    ondarvr Senior Member

    The sanding primer method works well when the surface is going to be painted, and on a small skiff it's most likely not going to be in the water for extended periods of time, and the paint choice indicates this.
     
  9. jorgepease
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    jorgepease Senior Member

    I don't know about this but am researching it myself. This was in the faq, sure you saw it but just in case.

    Can I use your 707-002 Surfacing Primer under the waterline on my boat?
    No! We have an excellent Vinyl Ester Blister/Osmosis System that we recommend. You can find the information under “Marine Applications”.
     
  10. fallguy
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    fallguy Senior Member

    If you use the right epoxy, wouldn't you post cure prior to any paint? That is what I am planning. I was told to fill and fair epoxy and post cure it at 140F for 2 hours to avoid any print through. Gotta build a boat cooking oven.. Personally, I would avoid painting a primer coat until after filling any open weave, fairing, and post curing, but I am no expert. In fact, I plan to see what the experts have to say about my post. Good luck.
     
  11. groper
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    groper Senior Member

    Its all been lost in translation... theres nothing wrong with what you suggested - the only problem is other people's interpretation of it.

    You domt intend to use the primer as a finish coat, you dont intend "substitute " a primer for a gelcoat, or any other bizarre ideas - your simply using it to pull a hull from a mold, using an epoxy laminate, which you will later paint in 2 pack to finish the hull of an 18ft skiff. Whats the big deal?
     
  12. rxcomposite
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    rxcomposite Senior Member

    Groper has a point there. If assuming you have a perfect mold surface and you pulled a part with sanding primer, then sanding to the fiberglass substrate would most likely form an imperfect part.

    The sanding primer is bulky and is used to fill up cosmetic or imperfections on a part. It fills up the the hills and valley. So just pull a part and spray it with a sanding primer, sand, then paint.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Most epoxy formulation do not need post curing to reach optimum strength unlike the vynil ester.

    Epoxy reaches its max strength at room temperature curing. If you use vynil ester and you want the same strength as epoxy, post cure it.
     
  14. fallguy
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    fallguy Senior Member

    I don't want to twist this thread up, but any good epoxy formulation used for foam ought to post cure for added strength and so it won't melt like butter in the tropical sun. Not all epoxies are designed for room temp full cure, and those that are won't perform as well in hot climates (from my read).

    https://www.systemthree.com/blogs/epoxy-files/83509828-postcuring

    Based on the link; a post cured epoxy should perform better at higher temp.

    I intend to post cure and I would not want to have primer on the boat before post cure. It might be fine; just wouldn't want it.

    I was interested in this thread because I had considered building without gelcoat as well. With all the problems of gelcoat and the successes of catalyzing paints; along with the additional weight of gelcoat; the only downsize of paint is gelcoat can handle a hard scratch better, but not sure it is worth the weight. So, I support the op's idea, but not sure I like the how.

    And if I didn't say it, I'm no expert.
     
    Last edited: Apr 16, 2017

  15. groper
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    groper Senior Member

    I did not say to use a primer for a mold release either.

    And you shoukd also read carefully what rx composite said. Hes a composites engineer so kinda is an expert...
     
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