Deck fill

General Discussion
1

I’m adding a few pieces of hardware to the deck of my F36 sloop. I’ve read the proper technique is to drill oversized holes, fill to maintain integrity of cored hull, and then drill proper sized holes in the filling. What is/are the best products to use to fill the larger holes?

2

Do not enlarge the deck fill hole.

  1. Remove the deck fill

  2. Tape over the bottom of the hole with waterproof tape

  3. Use a Dremel tool or an old hex key chucked into a drill to remove any wet core material or a minimum of 1/2" back from the hole edge between the upper and lower fiberglass skins.

  4. Mix up a "peanut butter consistency of epoxy resin and filler and fill the void you created and the opening with the epoxy mix.

  5. After the epoxy has cured redrill the deck fill opening and install the fill.

1 Like
3

Personally I prefer to overdrill and fill than to try and scrape out the balsa from inside a smaller hole. I just don’t think I’d get all the balsa out to be able to get a clean bond between the fill and the existing glass.

However I don’t drill the larger hole all the way through. Here’s what I do (a lot of steps but I’m just being overly detailed):

  1. If you care about your nice decks you probably should tape off the area with a layer of tape before you start. You can just drill through the tape and cleanup is much easier. I always forget this step and end up trying to wipe up drops of epoxy as it cures on my decks
  2. Drill the right-sized through holes for the fitting all the way through the deck. These holes will provide a template for the larger holes and will allow debris to escape
  3. Using a drill bit that is twice the size (ie, a 1/2" drill bit if the fasteners are 1/4"), carefully redrill through the top layer of fiberglass, then slow the bit down and gently drill through the soft balsa core. Don’t drill through the bottom layer of fiberglass. This increases the surface area of the bond
  4. Clean out the holes using a vacuum or compressed air
  5. Tape the inside with masking tape to hold the epoxy in
  6. Mix a two-part marine epoxy. I have an FGCI store down the road so I use 4:1 FGCI marine grade laminating epoxy. Work quickly now!
  7. Mix in thickener (colloidal silica or glass microbeads both work well) until it has ketchup consistency. It needs to be liquid enough that it can settle into the voids and soak into the core a little
  8. Fill a syringe with the thickened epoxy. You want a syringe with a neck that is small and long enough to reach the bottom of the hole, but not so small that you can’t get the thickened epoxy to fill it. Use the syringe to fill all of the holes from the bottom up. The syringe isn’t required, but it does keep the mess contained and by carefully filling from the bottom up you can avoid bubbles and voids. If you don’t use a syringe, use a mixing cup with a fine pour spout and carefully pour epoxy into the holes without letting the stream touch the edges
  9. Watch for a few minutes and add more epoxy if necessary as it soaks in to the core
  10. Tape over the epoxy with masking tape (to stop dirt/bugs/yourself from getting into it) and wait until it fully cures
  11. Remove masking tape from both sides
  12. Redrill your holes using the right size bit. Take it slow; the epoxy tends to chip easily. Don’t worry if you don’t exit exactly where you drilled previously - as long as the entire new hole is inside the epoxy plug it should be fine
  13. Apply a small amount of butyl tape around the holes on the bottom of the hardware and around the undersides of the screw heads. Do not seal the interior side; if water does get in the holes you want it to be able to drain out rather than staying inside and rotting the core
  14. Fasten the hardware using 316 grade stainless hardware, a solid backing plate on the inside, standard washers, and lock nuts. McMaster-Carr is a good source for fasteners. Hold the screws still and only tighten the nuts on the inside - this preserves the butyl seal. Try to tighten the screws evenly rather than making one tight and then the next. Make it firm, but don’t over-tighten or you will squeeze out all the sealant or even crush the deck
  15. Use a metal scraper to clean up any squeezed out sealant around the edges of the hardware


    On backing plates: I typically use “Structural FRP” sheets from McMaster-Carr in a thickness equal to the fastener diameter. Cut the general shape out at home using a jigsaw and sand off the rough edges, but only drill one hole. After step 12, tightly fasten the backing plate temporarily in the orientation you want it on the inside of the deck. Then, using the new holes in the deck as a template, drill the rest of the holes in the backing plate. In my experience your deck holes will never be perfectly straight, so if you try and make a backing plate that perfectly fits your hardware you’re going to be in for a frustrating time. Make it fit your actual drilled holes instead.
1 Like
4

I just do what this guy says to do: Marine How To. His posts on electrical terminations are also worth a read.

5

I second all of that with an alternativenway to fasten the hardware.
Drill and tap the epoxy.