GRP pitliner help ?

Discussion in 'Fiberglass and Composite Boat Building' started by PitMan, Apr 5, 2019.

  1. PitMan
    Joined: Apr 2019
    Posts: 7
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    Location: UK

    PitMan Junior Member

    Hi,
    Apologies, I don't own a boat. I am here to ask a few questions, relating to making a GRP liner for a garage pit. I hope you don't mind ? It is essentially a boat, fixed into a pit, or maybe a pond, without the water (I hope) !!

    My plan is a GRP shape to fit, gelcoated inside, then concreted into the pit.

    My questions are mainly related to the mould making. I will be making a female mould, most likely, using MDF ? Would that be suitable ? Obviously the finish of the outside of the shape is not critical, so I will lay matting straight onto the wood.

    I guess, I could also make this using a male mould, then gelcoat the inside once released ? Would that be easier, do you think ?

    I assume, I'll need some sort of waxy release agent applied to the MDF, in order for the GRP to be 'released' ! What is the best material for this ?

    Thanks in advance. Wayne.
     
    Last edited: Apr 5, 2019
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    No apologies necessary. Even landlubbers are welcome here.

    MDF is often used in mould making. Melamine surfaced would be beneficial for flats.

    There are many release agents. Mould material, size and fiberglass lay up technique determine the most appropriate one to use.

    Some more information from you will help us give you better advice.
    What are the demintions of the liner?
    How much pressure must it resist? Dirt/water pushing inward or outward?
    How well wit be supported from underneath?
    Does it need to hold anything else up?
    Does it need to be chemical/oil resistant?
    Fire resistant?
    Which resin? Epoxy, polyester, vinylester or other
    Type, weave and weight of fiberglass cloth?

    Even a simple drawing or sketch would help a lot.

    Be warned. Sticking around midnight infection you with an expensive boat addiction.
     
    Dejay likes this.
  3. PitMan
    Joined: Apr 2019
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    Location: UK

    PitMan Junior Member

    Thanks Blueknarr.

    Dimensions are 2.3m L x 0.8m W x 1.2m D.

    It will drop into a pre existing brick built pit, then back filled with concrete, supported underneath by a concrete base. It will have an overlap before the top two brick courses. Then those two courses laid on top, around the edge, if that makes sense, to hold it all down securely. I guess it needs to resist most automotive oils, but, it's only for occasional DIY use.

    Can you recommend a plan of attack ?
     
  4. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Is this a one off? Or do you plan to make multiple and sell them?

    For a one off I think making a mold would be really excessive. Or maybe I'm missing something. You could just directly prepare the concrete surface and lay up fiberglass. There are concrete sealing epoxy and polyurethane coating systems, not sure if those are needed.
    It would be less work, less material and also seems better than leaving a gap or backfilling that gap between the fiberglass and the concrete. You don't need separate structural strength.

    For a concave shape it would make more sense to build a male mold since you want the inside surface to be nice I assume. Male molds are also easier to make and sand in the corners and lay up.

    I assume this is about catching oils and fluids so they don't seep out into the concrete / ground water? Or more about water seeming in and accumulating in the pit? If there is water pressure on the sides or from below you might have to take that into account so it doesn't "float up".
     
  5. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    A few more questions first.

    Ate you skilled at HVLP spaying?
    How much fiberglass lamination experience do you poses?
    Do you have access to vacuum pumps?

    Epoxy/polyester?
    Fabrics?

    Do the sides need to be parallel or can they taper in at the bottom?

    Using a female mould with those dimensions will be awkward.
     
  6. PitMan
    Joined: Apr 2019
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    Location: UK

    PitMan Junior Member

    Hi Dejay. Sounds like good advice, I never thought of directly GRPing the walls, yes, it is just a one off !! The problem I am having is water ingress into the pit, about 2ft water table. I own a couple of classic cars and the humidity is a problem, plus I have to pump it out when I want to use it. I have considered the problem of the 'floating' liner, so it will be anchored by two courses of bricks at the top of the pit.

    I guess it needs some strength to the floor, to absorb dropping tools, etc.

    There is a company that makes liners and they're not too expensive in the grand scheme of things, just over £1k. But, they're generic and I want something tailored, which would cost £kkk's.

    I'll investigate the preperation of the concrete for directly GRPing. I am assuming it needs to be 100% dry before doing it ? I am in North Wales, so dry weather is infrequent even in summer !!

    My reasoning for using a mould, was that I was also going to add a Visquin DPC layer before dropping in the liner to add an extra layer of protection and I want this to be a ONE TIME fix that 100% works. I have spent the last 3 years experimenting with various epoxies, etc and now I just want it done.

    Maybe I was missing something earlier, but is the Gelcoat applied to the mould first then the laminate to the gelcoat ? Creating a smooth inner surface with a male mould ?


    Blueknarr.

    I can spray, yes. I have a couple of compressors and can spray paint ok.

    I have little to no knowledge or experience using fibreglass. Apart from a couple of dodgy repairs on cars !! Ssshhh !

    I don't have immediate access to a vacuum pump.

    I'm not sure about the Epoxy/Poly/fabric question.

    I would like the sides to be parallel, but if it's easier, they could be tapered.
     
  7. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    I just did a cursory search but there seem to be coatings that work on humid concrete. There might be also stuff formulated for emergency boat repair on the sea, but others can probably tell you more. I don't have experience with fiberglass yet, just learning :)

    You could create an initial barrier coat that then presents a good surface for the fiberglass layup. You might get away with no fiberglass at all too but it should help.

    You'll probably find products and info about this searching for pool building or coating garage floors with resins.

    If you can put a dehumidifier in that relatively small space and cover it should get the walls reasonably dry too.
     
  8. PitMan
    Joined: Apr 2019
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    Location: UK

    PitMan Junior Member

    Thanks Dejay. I'll investigate this too.
     
  9. PitMan
    Joined: Apr 2019
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    Location: UK

    PitMan Junior Member

    Just a very quick Google. G4 Dampseal is the stuff to use as a primer for direct adhesion to concrete.

    OK. It looks like this may be the way to go.
     
  10. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    I have decades of experience as an industrial painter. I have never used any product that didn't have issues when applied over damp or uncured concrete. As have you will all those frustrating failed fixes.

    Some things to consider:
    • Epoxy retails for about five times the price as polyester
    • Epoxy is one hundred times as resistant to osmotic pressure than polyester
    • Epoxy usually has a longer "open time" to work into fabric
    • How will you lay resinated fabric under foot in the bottom of the pit your standing in?
    • Parallel sides won't slide off of mould.
    • How to get back fill around top lip?

    My recumend approach:
    • Use epoxy and three layers of 1700 series cloth
    • Lay up three 2.+M X 1.+M flat panels with a 5 CM flange on one long side.
    • Lay up one flat panel slightly larger than bottom.
    • All panels on the same melamine board (reused)
    • Trim and bed long sides
    • Trim and bed short sides
    • Tab vertical seams with four layers of 1700 cloth
    • Trim and bed bottom, tab with four layers of 1700 cloth
    • Fair and sand.
    • Paint with automotive urathane
     
    Last edited: Apr 6, 2019
  11. PitMan
    Joined: Apr 2019
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    Location: UK

    PitMan Junior Member

    Thanks Blueknarr,

    Your approach sounds like a plan. I know what you mean about failing products. I only want to do this once !!

    So, just to be clear. Basically create the four walls separately, then create the floor separately. All seams taped with 4 layers ?

    So, what is 1700 series cloth ?

    Auto paint as opposed to any sort of gelcoat ? - Does this fabric not need that ?

    With this approach, are you saying it won't need to be backfilled ? i.e. Applied tightly against the concrete walls ? Should I use some sort of adhesive here to give it a good bond ? What about the floor, obviously this needs to be totally solid ?

    Why not create a single moulded liner ? and drop it in. I'm pretty sure I can come up with a solution to backfill it. I could bed the floor on a wet mortar/concrete, then back fill the sides, maybe leave a section of the lip out at each side/end to do this and layer over it later ? What do you think ?

    Is there a reason to avoid doing it as a one piece mould ?

    My backout plan, should this GRP thing get too difficult is to demolish the 1970's original and rebuild it using a good DPC !!!
     
    Last edited: Apr 6, 2019
  12. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Yes

    A type of fiberglass cloth. 1708 is very common

    Gelcoat is somewhat finicky. Figured you have some experience with auto paints. They would work well enough. Not the fabric, but the epoxy saturating the cloth is quickly degraded by UV. So must be painted

    Yes. I suggest using tar or morter as bonding agents. The better the bond, the thinner and less expensive the panel's need to be.
     
  13. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Sorry about the split reply. My device is acting strangely and quit letting me edit.

    Moulding a freestanding box would be expensive. It would require very thick (lots of expensive resin and fiberglass cloth) walls and floor. It will be difficult to pull off of the mould. Remember how difficult it can be to separate two 5-gallon pails.

    An alternative plan:
    • Build plywood liner box
    • Cover plywood inside and out with epoxy and 6 oz twill fiberglass cloth
    • Reinforce corners inside and out with extra layers
    • Paint inside
    • Drop into wet morter
     
  14. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Yeah building the panels separately on a table is probably easiest. Then you just tape the seams. But I'd be worried about adhesion and water accumulating behind the panels and pushing in.

    Just out of curiosity, I know that those 1k polyurethane foam use water to cure and start the reaction. I would think there are polyurethane coats / gelcoats that don't mind dampness at all and at least bond mechanically to the concrete surface?
    I mean isn't this like a very common problem that is frequently encountered in various concrete construction projects? Or when you build a pool for example. Shouldn't there be a good solution to this? Again, just curious.
     

  15. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Yes. This is why an unattached panel should be stout enough to resist the inward pressure.

    Damp concrete physically disintegrates when sealed up under paint. I've seen apperearant solid concrete surface grow an 1/8 inch thick powder in less than a week. No coating will adhere to powder.

    Very common. Great pains are made to dry concrete prior to painting.
     
    Dejay likes this.
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