Guidance on Sanding Vacuum Bagged Hard Top

Discussion in 'Fiberglass and Composite Boat Building' started by fly186, Jan 3, 2017.

  1. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    I need to knock down some epoxy ridges and bumps that formed under the peel ply/vacuum bag where some tenting occurred around my vacuum manifold.
    The epoxy has cured for 5 days. I've washed the entire thing with warm soapy water and rinsed/dried it well.

    I was planning to use some 80 or 100 grit paper on my Dewalt random orbit palm sander to knock this stuff down. Any better ideas?

    Edit: 80 or 100 could take weeks. I just tried some 36 on a sanding block and finally made some progress. I'll need to be careful though.
    I'm also thinking of rolling on another coat of resin to add some more strength since I can see areas with voids under the glass where it looks like we didn't have enough resin on top of the glass when we laid the core down. I'm talking about the bottom surface that was against the mold not the top surface that was in contact with the peel ply.

    Here's a picture...

    topresized.jpg
     
    Last edited: Jan 3, 2017
  2. gonzo
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    gonzo Senior Member

    Don't use a random orbital sander, because the soft pad will not fair the surface. A hard block is the proper tool to use. You can simple get a piece of plywood and wrap the sandpaper on it. I usually buy rolls of self-adhesive sandpaper, but it probably doesn't make sense for a small area.
     
  3. fly186
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    fly186 Junior Member

    Yep, I'll need to use some 12" boards with pretty aggressive grit to knock down those high spots. The area to be sanded is only about 50 sq ft on the top of the core. The bottom is super smooth since the laminate was against the waxed whiteboard material that I used for the mold.

    Thanks.
     
  4. SamSam
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    SamSam Senior Member

    Try one of these paint scrapers.
    [​IMG]Sharpen it often with a file. This one has 4 edges, 2 smooth and 2 corrugated, the smooth edge is what you want.

    This type works well also, especially if there are odd shapes to scrape. They can be modified to any shape you want.
    [​IMG]

    When you file them, relieve the corners a little so they won't dig in.

    Scraping is a whole lot faster and easier than sanding especially for the type of stuff in your photo.
     
  5. fly186
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    fly186 Junior Member

    The scraper works really well on those ridges. I also went to Harbor Freight and picked up a straight line air sander for $35. If it survives this job it will be worth it. Also got 25' rolls of 2-3/4" sanding tape. 80 and 120 grit.
    Let the fun begin :D
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Your new inline sander might do okay with that, depending on soft the pad is. There's two styles of pad, most have a semi hard solid rubber, while some have a much softer foam like rubber. You want the hard stuff. For an inline sander to work well, you need a big compressor. If you're running the typical homeowner's 2 HP puffer, it'll barely spin up the sander and once slightly loaded will stall frequently, waiting on tank pressure to come up again.

    Scrapers can do the job, though care with gouges is a constant battle. On raw epoxy, I use a belt sander, with coarse grits first (24 - 40) then switch to 80 - 100, as I sneak up on it. This takes some care to do well, also, because the belt sander is infamous for removing way too much way too fast.

    The safe way is as Gonzo suggests and a block and paper. Start with something coarse enough so you don't make a career out of it, then fine it down as you can.
     
  7. SamSam
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    SamSam Senior Member

    That won't do any good as more resin will not add any strength and it won't penetrate to the voids under the glass. It's maybe possible to use a syringe to squirt some under there. Otherwise you have to remove some glass and patch it or if possible, ignore it.

    The scrapers are for the ridge type stuff and small bumps etc. Since the glass is so thin, with just one 10 oz cloth, I'd sand it as little as possible and then apply a high build easy sanding primer. It's much easier and faster to fair that compared to fairing epoxy and glass.

    It seems like a pretty light layup for a panel that big, but I could be wrong about that. I would be concerned about attachment points to the framework, that they are able to withstand compression of bolts or that screws wouldn't strip out. At times there can be a lot of uplift on tops, what with winds and speed. Rough water and pounding creates a lot of stress also.
     
  8. fly186
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    fly186 Junior Member

    Agreed - it is very light. I've decided to add a layer of 1708 on top in the center where the main support is required and a second layer of 10 oz on the bottom side to avoid compression issues.
     
  9. SamSam
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    SamSam Senior Member

    Avoiding compression issues usually requires a solid core in the area where fastenings are located. The most basic is plywood, but it can be a solid glass layup or some sort of putty type stuff that hardens, like Bondo, but not Bondo as no one likes Bondo, but like Bondo.

    I searched for official ways to remedy the problem and came up with this, which I don't know if it has those particular answers but is very interesting anyways. It only shows chapter 7, does anyone know how to find the rest of the manual?

    https://www.faa.gov/regulations_pol...raft/amt_airframe_handbook/media/ama_ch07.pdf

    .
     
  10. Nolan Clark
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    Nolan Clark Junior Member

  11. SamSam
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    SamSam Senior Member

    Thank you. Now I want to get an airplane!
     
  12. Scot McPherson
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    Scot McPherson Senior Member

    I used a 60 grit flap wheels on my angle grinder. You have to be very careful, because it takes off a lot of material if you "catch", but with practice and care, it's the best damn tool I have for fast fairing and surface prep for epoxy and epoxy primer paints.
     
  13. fly186
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    fly186 Junior Member

    I purchased a $59 Dewalt 4 1/2" grinder at Home Depot and used a 60 grit flap wheel as advised. It works really well and you need to be very careful but it makes quick work of bumps and ridges. If I had to do it again, I would buy a variable speed grinder because at 10,000 RPM it removes material almost too fast.
     
  14. upchurchmr
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    upchurchmr Senior Member

    A new coat of resin will not add strength. The strength comes from the glass.
    The only way to fix areas which did not get wet thru the glass is to remove the glass locally and bond on a patch.
    The patch needs to overlap the original glass lamination.
     

  15. Scot McPherson
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    Scot McPherson Senior Member

    upchrchmr is right, you need to grind the bubble out and lay a patch on. You need good overlap, not just a 1/4inch. I would go with a 4 inch square patch and fair/feather the edges back to the original lamination after it's cured. if you don't, you are going to risk delamination starting at those bubbles.
     
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