Mold gelcoat finish question

Discussion in 'Fiberglass and Composite Boat Building' started by Scuff, Dec 14, 2020.

  1. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    I finished one half of my mold and I have a question on two areas of gelcoat that had problems.
    In the first area the gelcoat is cloudy with what look like fisheyes in it. You feel these eyes with a fingernail. This area corresponds to a spot repair I made with duratec when the plug got a ding in it. I didn't measure the mekp when I did this repair and I'm wondering if I could have over catalyzed and that may be the reason? The plug is also cloudy in this area but not damaged. First pic is before removing the pva.
    The second is what I believe is alligatoring. This appears in an area that looks thicker than the rest due to the way I sprayed it.
    I posted pics of the areas. I'd be grateful for any thoughts on improving my results as well as repairing the spots. Thanks.
     

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  2. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    Pic of second area
     

    Attached Files:

  3. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    It's very difficult to identify the exact cause of the defects from the pics.

    About the only thing you can do is sand it off and respray it.

    You need to measure accurately, being off on the ratio can defeat any good efforts you made in building it.
     
    Last edited: Dec 14, 2020
  4. Scuff
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    Scuff Senior Member

    I was meticulous on the gelcoat and resin ratios .. the duratec touch up not so much. Some hard lessons on this one for sure.
     
  5. ondarvr
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    Location: Monroe WA

    ondarvr Senior Member

    The glug glug method is common throughout the industry.

    It's due to how forgiving polyester chemistry is, you can do several things "not so right" and still end up with something that looks OK...at first.
     
  6. Steve Clark
    Joined: Jul 2004
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    Location: Narragansett Bay RI

    Steve Clark Charged Particle

    Gators are typically caused by thin gel coat. The styrene in the laminating resin is intended to etch into the gel coat and thus bond. If the film is too thin, the film will melt and buckle. Like using too much Duco cCement on plastic models.
    I don’t know how it is, but I had suppliers drop off buckets of gel coat trying to get our business that would gator no mater what we did. I couldn’t believe anyone would sell stuff that didn’t work.

    If this is for short runs, don’t sweat the shine too much. The shape is what matters, so sand and fill to your hearts content. Wet sandpaper and buffers after the fact works fine.
    There are several ways to mold center boards and rudders, but it would be helpful to know what the size of the board is and what boat. Some don’t scale well.
    SHC
     
  7. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    The boat is 28' and the board is 60" long and 18" at it's widest. I've finally succeeded in producing the two molds. I'm pretty satisfied with the finish there's no alligatoring. The areas that did alligator on one of the failed attempts were actually thicker .. I'm thinking those areas hadn't fully cured when I laid down the initial layer of matt. This has turned out to be the hardest part of my build so far. I really underestimated the effort and level of detail required to produce an acceptable mold. I'm out of town for the weekend I'll post pics of the molds when I get back. Thanks for the input!
     
  8. Steve Clark
    Joined: Jul 2004
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    Location: Narragansett Bay RI

    Steve Clark Charged Particle

    How you build this depends on what structure you have backing the mold and how rigid the mold is.
    I assume you have alignment pins, balls or flanges.
    My recent blade molds have all been CNC cut out of MDF or UHMW a back surface parallel to the parting line.
    This allows me to clamp the two halves together using either laminated plywood platens or wide flange I beams.
    At the very least you need some external way to assure that the center of the part is straight and that the section has not bulged as you bond it together.
    I usually build up several plies of masking tape on the leading edge so I can tab the leading edge together after I release the part. This isn't something we could afford to do in production, but it makes a difference.
    In the simplest terms build either half and bond a core constant thickness in place. This is best done with a vacuum bag when laminating the skins.
    The core will be proud approximately parallel to the section shape.
    You then contrive to machine the centerline flat. A router with guides that hold the base above the flanges is one way, a power plane, grinder and long board is another.
    You need to clean all the laminate and gel coat off the flanges so the mold will close properly. If you are open molding, this is best done with something like a putty knife with a sharpened edge just as the resin is gelling. Timi9ng and technique is critical. also making sure you have minimum amount of laminate overlapping the flange. There is no need to pack lots of fiber into the absolute edges of the part, so being thoughtful of how you place the reinforcement is very helpful.
    It really doesn't matter as long as you get the two center lines flat. You want to avoid having to use thickened glue to bond the halves because that increases the force necessary to close the molds.
    I place ply of woven cloth on centerline to held transfer the sheer across the core and to reinforce the skinny bit at the training edge. Use plenty of resin and expect it to squeeze out all around.
    This isn't a time to be neat, put some plastic on the floor and let it flow.
    Re-wax the flanges before you bond.
    Use lots of clamps and wind them on evenly. If you can make sure one side is flat and straight and fastened to something big and stiff it helps.
    I have some lengths of I beam and some 2"square steel tubing that pretty much does the job for things I build.
    SHC
     
  9. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    Pic of mold. I made 4" flanges no locating dowels.
     

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  10. Steve Clark
    Joined: Jul 2004
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    Steve Clark Charged Particle

    Then reassemble the molds around the plug and drill probably six 1/2” holes that you can use for bolting the halves together before you start clamping. One hole in each corner and two half way down the long edges won’t be wrong.
    You absolutely don’t want things to start skidding around.
    SHC
     
  11. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    I like the use of masking tape for the leading edge. Simply remove the tape when done and use the mold to store the plug. How do you keep the tape from sticking?
     
  12. Steve Clark
    Joined: Jul 2004
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    Steve Clark Charged Particle

    The tape sticks to the mold before molding, but usually comes away with the part.
    SHC
     

  13. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Try appling the release agent on top of the tape
     
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