Fiberglass Hull Repair

How to make your bottom pretty. A guide to fiberglass work on small sailboats

  1. Bradley
    This article applies to all fiberglass hull repair, including work on Sunfish and Laser Sailboats.

    If you carefully follow the directions put forth in this article, you can have the nicest hull in your fleet. First, you must acquire the proper attitude about taking care of your boat. If you still pull your boat up on a dock without first checking for nails, screws and gravel, donít even bother reading this article. Your Sunfish only has about enough gelcoat for two hull rescue jobs.

    MATERIALS AND EQUIPMENT:
    1. Place to work for about 12 hours that can be washed down with water
    2. Gelcoat to match your hull and catalyst
    3. Acetone
    4. Nuffing compound (Dupont 101, Acme 50, 3M Super Heavy Duty, or a similar grit)
    5. Sharp knife
    6. Cardboard cut in a bout 10'' x 10'' squares
    7. Stir sticks (Popsicle sticks)
    8. Bucket and water
    9. 12 chunks of terry cloth (cut up old bath towels into 8 chunks each)
    10. Wet-sand paper in 320, 400, 500, 600, 800, 1000, and 1500 grits. If your scratches are particularly large or numerous you may also want 180 and 240 grit papers. For whichever grit you use first you will need about 5 sheets. For the other grits you will need about 1 sheet each.
    11. Gel Gloss or TR 500
    12. Maguire's Deep Crystal paste wax
    13. Squeegee ( I use a Thalco laminators squeegee but a good window squeegee will do the job)
    14. Pencil
    15. Cleansers (Comet or Ajax)
    16. Sanding blocks (I use a 9î block from an automotive paint supplier)
    DIRECTIONS:
    The boat needs to be supported upside down at a height where you can stand over it for many hours. You donít want to damage the boat or your back.
    1. Read all of these directions before you do anything to your boat. If you donít understand the directions, or if you have any trouble making the various steps come out correctly, take your boat to a professional.
    2. Wash the boat with a good cleanser and then clean it again with acetone.
    3. Every scratch that you can feel will need to be filled. Use a sharp knife and lightly re-gouge the scratches. Your scraping should create dust, not chunks. You must have a freshly roughened surface so your repairs can adhere to the grooves.
    4. Blow away the dust and wash the boat again with acetone. Make sure that you have removed every last particle . If your repairs are made over a dirty surface the repairs will stick to the dirt but not to your boat.
    5. Test the gelcoat to make sure it doesnít set up too fast or too slowly by mixing a small amount on your cardboard. Dribble a half dollar size disk of gelcoat off a stir stick. Then dribble a pea sized amount of catalyst into the center of the gelcoat and mix it as well as you can. Scrape, wipe, smear and swirl that puddle of material until you are sure that it is mixed. Play with the stuff for 15 minutes to make sure that it isnít becoming hard too fast--if it sets up too fast you wonít have time to apply it to the boat. Catalyzed gel coat can get really hot. Set the cardboard down somewhere where it canít start a fire if it ignites. Go away for an hour. When you get back the surface of the puddle should be sticky but the material underneath should have hardened. To see if it is hard, fold the cardboard. The puddle should break. If your test batch matches this description, you have learned how to mix gelcoat. If the gelcoat didnít cure, start with a new batch and either add more catalyst or find a warmer place to do your work.
    6. Once you have learned to mix the gelcoat, mix a fresh batch. Using your stir stick or an artists brush, paint each of the scratches. Just try to fill the gouges level with the boat surface. This may take a couple of passes between which you need to go away for a soda or a beer depending on your age and preference. Do not wait more than a couple of hours, period.
    7. After you are convinced that all the scratches are filled, put on one more coat. This last coat can be mixed with a little extra catalyst to hurry along the process. Gelcoat does not fully cure when it is exposed to moisture in the air, so this last coat is to help cure the sticky part of the previous layer. This is a good place to stop for the night to allow the gelcoat to harden.
    8. Fold one of your terry cloth pieces and soak it with acetone. (Itís nice if your terry cloth is a contrasting color to the hull.) Wipe off the part of your gelcoat that will soak easily free. Keep wiping until the towel shows no more color coming off. If you have lots of scratches this may take a couple of towels. You will also need a fresh towel to wash the sticky stuff off your hands.
    9. It is time to use sandpaper. Sandpaper can follow the contour or the surface and remove the softest part, or it can ride gently over the surface, trimming off only the high spots. YOU MUST FOLLOW THESE RULES:

      Use Sharp Sandpaper!

      Do Not Press Down Hard!
    10. It is time to sand off the extra gelcoat. Use sharp sandpaper. Do not press down hard. Ideally, you will use the finest grit that your patience will allow and a wood block. You will sand away the excess gelcoat without ever touching the adjacent pristine surface of your boat. I usually accomplish this task with as tiny a wood block as I can hold and a lot of brand new sandpaper. The sandpaper should not touch anything other than your repair until the excess is almost totally removed. Be patient. Use sharp paper. Do not press down hard. I recommend 320 or 240 for this step. Stop often and look at how you are doing. Remember that you do not want to sand anything except the stuff sticking out above the scratch. Occasionally you should use your squeegee to dry the work area. Stop. Look and feel how you are doing.
      Let me digress a moment here. What I just described can be more easily accomplished using dry sandpaper. I use the white or gold type of paper. However, if you choose to sand dry you will be creating a lot of dust and adequate protection is necessary. You will need a protective mask and the work area will be coated by your dust. The advantage of dry sanding is that you can wipe away the dust and see exactly where you have sanded.
    11. When all the repairs are flat and level with the hull surface it is time to begin working on the whole hull. Do not begin sanding the whole hull until you have finished sanding all the individual scratches. You need the shiny surface of the hull as a reference until the heavy sanding is finished so that you donít make the surface wavy.
      Now itís time to get all the ripples off the entire hull. As long as you can smell styrene inside your boat the plastic is shrinking, becoming more crystalline and just plain getting uglier. You want to remove all the tiny ripples that your boat has developed as the plastic has continued to cure since it left the production mold. Remember that you are not attempting nor are you allowed to change the shape of your boat. This is a cosmetic repair, not a speed enhancement.
      I usually start the whole hull job with 320 paper. Using a soft rubber block, I sand at 45 degrees to the centerline until the entire hull is a consistent, dull finish with all of the sanding scratches parallel. The reason to keep all of the scratches parallel is so that when you switch to another grade of sandpaper, you can sand in a different direction and know when you have removed all of the scratches from the previous grade.
      Itís graffiti time. Use a pencil to make marks all over your hull. When Eric Faust does this part he creates cartoons and other nonsense, but lazy guys like me just scribble. The object is to make enough pencil marks so that it is easy to tell where you have and have not already sanded. Turn your sanding to the other 45 degree angle and shift up to your next finer grade of paper. Sand away all the pencil marks and then inspect your work. All the sanding scratches should run in the new direction. In areas where the old scratches still show, pencil and sand again.
      Repeat the penciling and sanding with 400, 500, 600, 800, 1000, 1200 and 1500 grit papers. You can skip grits or stop at a heavier grit but your boat will not be as shiny if you donít use the whole series. To make the bottom of your hull heavenly, you have to sand it thoroughly.
    12. Now, smear buffing compound all over the hull and with one of your clean pieces of terry cloth, rub it until you are sick of rubbing. Rubbing compounds work a lot like sink cleansers--the more you stroke the surface the better the final appearance.
    13. Using water and a clean rag, rinse off the remaining compound.
    14. Apply Gel Gloss according to the directions on the can. If you are not paranoid about the possible loss of boat speed you may also want to wax your boat. When you go to a really important event in fresh water, you may wish to remove the wax with a strong detergent. I think that a hull coated with Gel Gloss is faster than a good clean hull in brackish water. I donít like to have crud stick to my hull--I think it probably slows my boat down. When sailing in Lake Pontchatrain a J-24 coated with Gel Gloss will remain clean for a week while boats without Gel Gloss acquire a nasty yellow coating. Make your own decision.
    • A final note: the gelcoat on your hull is only thick enough to endure this process a couple of times. If you arenít sure that you will take care of your boat starting immediately, do not waste the repair opportunity now.
    • Next time you are approaching the starting line you can strike fear into the competition by having your boat glare at the other sailors.