How to see imperfections in a workpiece

Discussion in 'Fiberglass and Composite Boat Building' started by aaronhl, May 13, 2020.

  1. aaronhl
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    aaronhl Senior Member

    In my various fiberglass/epoxy projects, I find it difficult to see all the tiny imperfections like scratches and low areas because of the color differences/different type of epoxy in the piece. I'll use a filler that's white, fairing that's tan, and the work piece may be previously painted/gelcoated a different color.

    I think I have all the imperfections worked out... just to see them after I paint the part (with spray paint)...Once I spray paint, it's hard to sand, then rework with epoxy.

    Any tricks or ideas to see the work piece better for a smooth, almost blemish-free finish??
     
  2. ondarvr
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    ondarvr Senior Member

    What is it you're making. There are several options.
     
  3. aaronhl
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    aaronhl Senior Member

    a variety of things, like a plug for a mold, a repaired fiberglass section on the boat hull etc. I have also found it challenging on a recent project when I had to epoxy a part into my boat's lower unit only to find all the imperfections after I spray painted it black. I plan on making another mold soon so don't want to see the same problem. hope you guys can understand exactly what I am referring to...without spraying an coating like duratec/epoxy spray coating or something like that how I can I see the imperfections before hand. the molds rip the spray paint right off so I dont feel like I can use that in any part of the process. My molds are of course curved/angled so spraying seems like the only option
     
  4. ondarvr
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    ondarvr Senior Member

    Without using the correct methods and products you can't easily detect and eliminate surface defects.

    That's why these products exist, and why fabricators use them so much.
     
  5. aaronhl
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    aaronhl Senior Member

    I feel like I would have to spray several times before I had the finish piece, since you may not see all perfections with only one spray. Would you say spraying a sandable coating is the best way? Any alternatives that may not be as ideal?
     
  6. ondarvr
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    ondarvr Senior Member

    Frequently people use an automotive type primer that can't be applied nearly as thick as a polyester sanding primer. This results in sanding through and more coats needing to be applied.

    You can apply something like Duratec primer at 30-40 mils thick, or triple that with the high build products.

    This allows you to only make one application and bury the defects deep enough so you don't sand through.

    For plug building you can then sand and polish the primer to eliminate a top coat if you don't require an ultra high gloss finish.

    Some people like to get the best gloss possible on the plug so the mold doesn't require more work. Others do the surface finish work on the mold.
     
  7. aaronhl
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    aaronhl Senior Member

    OK That's makes sense, Is duratec considered a paint type of solvent that gums up the sandpaper easily? is a spray like duratec sandable like an epoxy fairing mix?
     
  8. ondarvr
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    ondarvr Senior Member

    It's much easier to work with compared to epoxy.

    It cures tack free, so no clogged paper.
     
  9. gonzo
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    gonzo Senior Member

    The usual method to find imperfections is to lightly spray a mist of contrasting color. After finishing sanding, if you can still see color in any place, it is a dip or pinhole.
     
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  10. Blueknarr
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    Blueknarr Senior Member

    A few things that have helped me a lot.

    Clean the piece so that no dust is in the scratches thus hiding them.

    Low angle viewing and lighting. Maximize the shadow's potential to show hills and valleys. Look from many directions switching the light source from left to right and back again.

    Try non-prescription magnifying reading glasses.

    As Ondarvr pionts out- prime, prime, prime. One of the jobs of primer is to provide a uniform surface (color, density and porosity).

    Let your paint dry longer! Unless you are using household latex paint (which is always gummy) clogged sandpaper is an indication that the paint isn't cured.
     
  11. waikikin
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    waikikin Senior Member

    I have one word... actually two .... guide coat...

    Everyone gets "snowblind on large surfaces,
    there tricks like the use of guide coat for critical inspections tabs of masking tape help, setting a grid pattern to a large surface...
    and once you've done your best and considering that most are our own worst critics the next
    Do this
    We call it guide coat
    also set up a grid pattern to work along sequentially & top up as required.
    Use battens or splines to indicate fill required- notes in light pencil help
    also it usually take three hits or screeds to get a surface fair unless you overfill and have lots of labour to work on the surface
    We call this guidecoat

    also set out grid pattern so the surface can have scrutiny in a logical progressive fashion
    also it usually takes three screeds of filler to achieve fair, once fair & primed a few small top ups will be required.
    Also once you've done your best just own it, if anyone picks on it either give them a two word answer or thanks "at least you're leaving someone else alone at the moment" & leave it at that...

    Jeff
     
  12. fallguy
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    fallguy Senior Member

    I use 3M guide powder. It concentrates in low areas.
     
  13. cy fishburn
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    cy fishburn New Member

    I have used a "layout fluid" called Dykem Steel Blue. Dilute 1:3 or so with acetone, or to suit. Apply with cloth and sand/fill. Also works great to find pinholes.
     
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  14. aaronhl
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    aaronhl Senior Member

    awesome some good input above

    any recommendations for a guide coat to mist on? I see the 3m guide power it is a brush application
     

  15. aaronhl
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    aaronhl Senior Member

    Thank you for that quote it's hard to forget that sometimes
     
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