Making a mould...

Discussion in 'Fiberglass and Composite Boat Building' started by JohnMarc, Feb 11, 2019.

  1. tpenfield
    Joined: Dec 2016
    Posts: 144
    Likes: 9, Points: 18, Legacy Rep: 10
    Location: Cape Cod, MA

    tpenfield Senior Member

    Usually the PVA alone is enough. The only problem is PVA is best applied by spray gun rather than brush, as it can leave brush streaks in your mold. Wax is more tedious, because you will want/need about 6 coats of wax, polishing in between coats.
     
  2. JohnMarc
    Joined: Feb 2018
    Posts: 78
    Likes: 3, Points: 8
    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    Awesome thanks, for a plug I am only using once just not worth the effort, so PVA the way to go, yes I have a spray gun etc. so will spray it on.....
     
  3. fallguy
    Joined: Dec 2016
    Posts: 1,668
    Likes: 86, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I wax and pva.

    Pva is water soluble. Wash part with soap n water when done.
     
  4. JohnMarc
    Joined: Feb 2018
    Posts: 78
    Likes: 3, Points: 8
    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    Yes was actually thinking that would be the way to, was looking at the Plug and have other uses for that size. so will do the full treatment I see the waxing is done up to ten times.....:rolleyes: will see how we go.....
     
  5. kapnD
    Joined: Jan 2003
    Posts: 437
    Likes: 29, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    PVA is not fussy stuff to spray, and a thin coating is all that’s required, so a pump bottle that atomizes nicely works well, at least for small parts. Make sure it cures fully to avoid the dreaded alligator effect!
     
  6. fallguy
    Joined: Dec 2016
    Posts: 1,668
    Likes: 86, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I tried the pump method and was not a fan because the pump didn't have enough air for my big part.. I am using a regular sprayer with air compressor.

    What is alligator effect? I probably have seen it; never heard it named.
     
  7. kapnD
    Joined: Jan 2003
    Posts: 437
    Likes: 29, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    It’s when your gelcoat “wrinkles” from being applied over uncured PVA, or other contaminant.
     
  8. JohnMarc
    Joined: Feb 2018
    Posts: 78
    Likes: 3, Points: 8
    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    Thanks for the heads up, I assume PVA cannot be left for too long (drying time) prior to use, within reason of course.....
     
  9. tpenfield
    Joined: Dec 2016
    Posts: 144
    Likes: 9, Points: 18, Legacy Rep: 10
    Location: Cape Cod, MA

    tpenfield Senior Member

    As stated above, the PVA needs to be fully cured before you apply the gelcoat. So, drying time and then some.
     
  10. fallguy
    Joined: Dec 2016
    Posts: 1,668
    Likes: 86, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    When you spray pva; it is best to have your vac bag glue strips down with the top/paper still on.

    Otherwise, you spray and then attach the strips to a surface that is too weak and then the pva under the glue strip cracks and you lose vac on the job.

    I forgot once. And the job got no vac since we ran out of time (bigger part).
     
    Last edited: Feb 14, 2019
  11. JohnMarc
    Joined: Feb 2018
    Posts: 78
    Likes: 3, Points: 8
    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    So have now finished my topcoat on the plug, I had to redo the paint as it set too quickly, we having a particularly hot day. I am going to take this lesson into the resin, am I right in saying a wee bit less hardener or mmmm catalyst (I think it is called) will delay the curing process but not have any other detrimental effect to my product.
    Yup thanks good point .... I would have walked right into that one.
     
  12. JohnMarc
    Joined: Feb 2018
    Posts: 78
    Likes: 3, Points: 8
    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    I haven't really had any feedback to the point I raised in an earlier post, am I correct in assuming that for my female mould, in order to get the smooth finish on my hatch, I would need to use a gelcoat on my plug first before laying the fibreglass? The second question then was I assumed that would mean I would use poly as apposed to epoxy? This is because the the epoxy vs. poly argument and compatibility with gelcoat leaves me a wee bit lost .... there are many varying views as to wether I can use gelcoat with an epoxy resin, so thinking stick with Poly for the female mould .....????
     
  13. tpenfield
    Joined: Dec 2016
    Posts: 144
    Likes: 9, Points: 18, Legacy Rep: 10
    Location: Cape Cod, MA

    tpenfield Senior Member

    You will want to use gelcoat as the first layer over your plug to get a nice smooth, fiber free finish on the female mold. You can use epoxy over poly, but not poly over epoxy. so, if you put down poly gelcoat, you will be fine to use epoxy in the laminations to follow.

    The problem with using both poly and epoxy is that when epoxy cures, it leaves amines on its surface and that prevents poly from curing. Try it out on a small test piece . . . put some epoxy down and let it cure, then put some poly over it. The surface of the poly that meets the epoxy will stay liquid, even though the poly mostly cures. You will be able to pull the hardened poly right off of the epoxy.
     
    Last edited: Feb 14, 2019
  14. JohnMarc
    Joined: Feb 2018
    Posts: 78
    Likes: 3, Points: 8
    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    Okay really appreciate your repose, much clearer, but ultimately I am planning to use epoxy for the main build which is my boat. I am assuming then that a gelcoat cannot be used, what then does one use as a finish over an epoxy infused hull for example. For the helm and other bits and pieces do i have to use poly because gelcoat seems to give the most durable finish?
     

  15. fallguy
    Joined: Dec 2016
    Posts: 1,668
    Likes: 86, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    A lot of epoxy builders avoid gel coat altogether for 2 part epoxy paints.

    Paints offer many advantages for the home builder. Stench control is one of many.

    My build is all epoxy; no gelcoats. However, some of my moulds used poly mould making materials.
     
Loading...
Similar Threads
  1. Felix Gruter
    Replies:
    15
    Views:
    1,559
  2. Raggi_Thor
    Replies:
    11
    Views:
    6,803
  3. scotboats
    Replies:
    2
    Views:
    2,149
  4. PROPGUNONE
    Replies:
    4
    Views:
    401
  5. fiberglass newbie
    Replies:
    3
    Views:
    407
  6. Nick.K
    Replies:
    7
    Views:
    736
  7. sailsherpa
    Replies:
    2
    Views:
    749
  8. missinginaction
    Replies:
    3
    Views:
    1,861
  9. pavel915
    Replies:
    6
    Views:
    4,226
  10. tugboat
    Replies:
    87
    Views:
    8,786
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.