Tuesday, August 19, 2014

Stringer Replacement

The seats and floor boards had been providing some degree of structure for the stringers, although by design it should have been the other way around.  Once I removed those pieces of wood, the remaining foundation was found to have de-laminated quite badly.  A light touch of a finger tip was sufficient to impart a 1/4" wiggle to the frames.  Some of the cracked tabbing is visible in the picture below.

I also couldn't help but notice that most of the screws I removed were no longer biting into anything solid.  They were completely stripped out.  With that in mind, I decided to replace the active cores of the stringers as well using the old ones for rough template lines.  The original material is (thought to be) western red cedar, and having no better ideas, I plan to replace them with the same.

The first step was to come up with a materials list, and get all my supplies together...

  1. Sanding Supplies
    1. 50 grit discs for 4.5" grinder
    2. 80 grit discs for 4.5" grinder
  2. Fiberglass Supplies
    1. Fiberglass cloth
    2. Epoxy resin + hardener
    3. Colloidal silica thickener
    4. Mixing sticks
    5. Mixing cups
    6. Chip brushes
    7. Spreaders
    8. Acetone
  3. Wood
    1. Western Red Cedar - not sure of standard dimensions available 
      1. Assume 8' long, nominal width:  96" x 3" x 1":  2/board = 8 boards
      2. Assume 8' long, 6" width:  96" x 6" x 1": 4/board = 4 boards

I tend to over-estimate, as it's more expensive for me to have to make a return trip to the store than it is to have a little scrap left over (or extra material in case I goof up!). Assuming 8 stringers per side, 16 total gives me plenty of margin for error.  Each stringer needs to be 2" high and 7/8" wide, so I need to purchase 1" thick boards and plane them down.

High Level Plan

  1. Use multi-master flush cut-off tool and 4.5" angle-grinder to surgically remove the existing stringers with as little damage to profile as possible.
  2. Grind surface where new stringers will go, including tabbing width, to clean, bare fiberglass.
  3. Cut out new stringer following profile from old.
  4. Laminate one layer of 8oz. or 10 oz. cloth as a weight-distributing base for the stringer.
  5. Bed new stringer in same position as original in epoxy thickened with colloidal silica.  Make 1/2" radius fillet all around.
  6. Once cured, remove amine blush, then grind any imperfections or epoxy spillage.
  7. Lay up two layers of 6oz. cloth over stringer with 1/2" reduction / taper per side.
  8. When kicked, paint on 1-2 coats of neat epoxy to fill cloth texture.
  9. Once fully cured, remove amine blush, then grind any surface imperfections flush.
  10. Prime repair area with Interlux Primekote 2-part epoxy primer.

Removal of Damaged Stringers

This went mostly according to plan.  First I took many careful measurements to ensure I could put the stringers back as they were intended.  I  then used my Harbor Freight Multi-Master knock-off to cut the tabbing on each stringer.  I used the multi-speed version of their tool, which felt more substantially built than the single speed unit.  I have to say that each time I've used it I have been impressed with how well it works.

The slots in the fiberglass had pretty significant lips at the cut lines, as well as depressions where the stringers had been.  I try to avoid using a grinder whenever possible because it just sends dust everywhere, but when there is a lot of material to remove, nothing else works as well.  So, out came the grinder with a 60 grit disc.

It took me about four hours to completely grind and rough-fair the remainder of the stringers, and by the time I was done, I was pretty sick of grinding.  I stopped after every other stringer to vacuum up the dust, so I perhaps could have saved some time if I'd been less neat.

I also ground out a few places where the laminate had separated from the core so I can relaminate, and I cleaned up a few places where prior repairs had been done in less careful manner than I prefer.  So now, I'm ready to start filling in holes.

Repairing the Laminate

Each of the slots that had housed a stringer is now ground out in a concave manner.  I wanted to allow the stringers to ride a bit higher, just to help keep the water a bit less likely to pool on them as before.  TO fill the cavity I will lay down one narrow layer of 8oz cloth as a foundation, and then one wide 6oz tab to distribute the load.  This will not only add some rigidity, but also help to fair the bottom a bit.  I know, only I would care about fairing the bottom.

More to come:  I will continue updating this project page as I make progress.

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