Fiberglass grinding mistake correction

Discussion in 'Fiberglass and Composite Boat Building' started by jsfindley, Aug 6, 2018.

  1. jsfindley
    Joined: Aug 2018
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    Location: 38375

    jsfindley Junior Member

    I am a noob at fiberglass and boat repair. I have an old 1978 RK 20 I have been working on to learn these skills. I was using a random orbital sander to remove the paint and gel-coat down to the fiberglass for a repair. This was taking forever, so I did some looking through forums like this one. I found that people were using 60 or 80 grit flap disks on an angle grinder. I tried this, but I think I did it wrong. I hope I didn't ruin the hull. I only have a 4" grinder. I used the 80 grit flap disk. I didn't notice through all of the caked up dust, but after I finished grinding away the paint, I found that there were ripples in the fiberglass. I guess I should use fairing compound or somehow sand it the slow way so that it is smooth and lay an additional layer of fiberglass in this section. I'm not sure what to do. Can someone help. Thanks.
     
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member



    Welcome,

    Breathe a sigh of relief. As long as you didn't use a blowtorch, it can be repaired.

    Picture with a thousand words. Unit then all I can do is guess.

    You probably needed to remove some glass anyway. Glass is typically ground out at 12:1. So 12 inches wide for every inch down.

    Probably smooth ridges with soft sander before re-glassing.

    Normal order of materials:
    Glass, fairing, gelcoat/paint.

    Unusual to want fairing in the middle of glass work.

    Good luck
     
  3. jsfindley
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    jsfindley Junior Member

    I know fairing shouldn't come in the middle. I wasn't sure how to get the fiberglass back smooth easily. I was sort of trying to brainstorm and think outside of the box to find a solution. I am just not sure what to do next. I'll send a pic when I get home from work today.
     
  4. Blueknarr
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    Blueknarr Senior Member

    Cool
    Didn't mean to offend. As a FG repair professional, I often am asked to redo bondo only non-repairs.

    But there are times when fairing compound should be used bellow glass cloth.

    Should be able to give more comprehensive advice after seeing pix
     
  5. jsfindley
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    jsfindley Junior Member

    This is a picture of the area. There were a couple of big gashes into the fiberglass through the gelcoat. I grinded them all out. 20180806_190347.jpg 20180806_190337.jpg 20180806_190348.jpg
     
  6. jsfindley
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    jsfindley Junior Member

    There was no offense taken. I defer to your expertise on this matter.
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What was the purpose of grinding well away from the damaged area ?
     
  8. jsfindley
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    jsfindley Junior Member

    It was a big cut into the surface. I assume I made a mistake. I'm asking how to fix it.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Have you repaired the damaged area ? ( not the "damage" caused by grinding).
     
  10. jsfindley
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    jsfindley Junior Member

    I just grinded/sanded past the damage. I figured I should put a layer of fiberglass over the area to replace the removed material. Please tell me what to do next.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You have to repair the hull damage, that caused you to embark on this exercise, in the first place. Obviously you want to end with as close to an "invisible repair" as you can, the first priority has to be structural soundness, then you can concentrate on the cosmetics. You might need some close-up photos of the original damage, and your own added damage, to get a meaningful answer, as advice how to best proceed. You can't just glass over damage like adding a band-aid, without it looking an eyesore.
     
  12. jsfindley
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    jsfindley Junior Member

    Here's the situation. I realize that I got carried away with the grinder. I did grind and sand down past the initial damage that I was attempting to correct. Now I want to do what is necessary to get the hull smooth, as my grinding was somewhat inconsistent and I made a lot of waviness in the fiberglass that was not there before. Surely there is a way to fix this. I'm sure it's pretty elementary. Just consider me as being someone who knows nothing and even if it is the most simple solution that anyone should have thought of first, I probably missed it. Do I keep sanding with my random orbital sander and eventually get it smooth after some time. Is there some other thing I should do? After I eventually smooth the surface, shouldn't I replace the removed fiberglass for the sake of structural integrity? Also, there was never a through hole. I probably did overkill on this and sanded too much. I was thinking I needed to get the hull, at least in this place, down to the fiberglass to check for any cracks or problems under the surface since this was a nasty gash at one point and since it is an old boat. I want it to be in good shape when I'm done. I'm not in a hurry. I'm not asking for a quick fix. I just want the best and most efficient way to get myself out of the mess I made. I'm using this boat to learn on so that if I ever have problems with my nice boat, I will know what to do. I'm asking for advice.
     
  13. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Good morning

    I tried to analyze your pic. Their resulution on my screen did not allow my to see what I was looking for.

    When I use flapper disks I often get ridges as you describe. They are caused by the too stiff backing plate. I prefer grinding with soft plates. This is pneumatic but still works on electric. See how it flexes to almost conform to surface. Leaves far fewer ridges.

    How much glass thickness did you remove?

    Have customers. More soon
     

    Attached Files:

  14. jsfindley
    Joined: Aug 2018
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    jsfindley Junior Member

    @Blueknarr I believe I only removed maybe a millimeter below the paint, not much. The gash wasn't very deep, but it was long and slightly into the glass. Thank you for your reply... the most informative and actually addressed my the most by far.
     

  15. Blueknarr
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    Blueknarr Senior Member

    I'm back
    As the outside corner of the flappy disk is worn down it well behave better. Grinding on concrete can speed the process. Time working with the tool will teach you how to finesse things. No way to speed up that process.

    80 grit probably didn't cause too great ripples to have too worry about. Sanding with less aggressive tool won't hurt.

    About 2/3s aft and 1/4 down is a small constellation of 3 dark spots and one yellow one. Could you send a close up of this area?

    I am greatly concerned about the overall color of the glass. I would like to see everything the same darkness as at transom.
     
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