You asked - tough to explain so here it goes
NOTES:
I recommend this process only if you have a headliner and not finished gel coat underneath because it would be easier to cosmetically repair the top deck with non-skid than gel coat from underneath.
This is very messy so cover the areas around you well.
Wear a respirator mask. When resin/epoxy kicks in a closed area like a boat cabin the fumes can be pretty bad.
Wear protective eyewear.
MATERIALS: (the first section can be ordered from DynaGrout.com - nice guys, reasonable prices)
fiberglass Matte
fiberglass cloth
Balsa Core
3" laminating roller
Resin or Epoxy (your choice)
Structural thickener (your choice)
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Chip brushes
plastic putty knives
small paint roller and small paint roller pan (disposable)
Acetone (only effective way to get this stuff off of you - not the healthiest
so if anyone has a better recommendation I am all ears)
small shop vac
small funnel
hemp rope (cheap rope from Home Depot - thick diameter, not twine)
6 mil. or heavier plastic - clear so you can see
duct tape
resin paper and blue tape (to cover underneath the area you will be working)
PREP:
Cut out fiberglass skin underneath and remove the damaged core. I recommend a router or rotozip to cut the glass. Be sure to only cut the underneath and not go through the top skin.
Sand/fair underneath of top skin.
If you have areas/crevices that you will have trouble getting a large square of balsa to fit, no worries. We'll get to that, just clean out as best you can.
LET'S BEGIN:
NOTE:
If you are working on a large enough area you will do this is stages as you have to work within the working time of the resin/epoxy you are using. Do each stage (labeled by number) at a time. e.g. Stage 1 until done with all sections, then stage 2., and so on.
For those hard to reach, small crevice areas we will address those first. The object is to fill enough of those areas that the remaining square will fit a piece of unbroken balsa.
1. Mix a pot of thickened epoxy/resin to a peanut butter consistency. Not running. Completely static. Push the thickened epoxy/resin into the crevice. If space allows jam a piece of the balsa core in. If there are multiple, then line up the pieces. Once satisfied, let it kick. In the below picture you can see where I had to do this.
2. Measure area for a section of balsa core. Cut to size.
Measure out a piece or two of matte glass. Cut to size.
(Create vacuum bag) Cut a piece of plastic larger than the area. Cut a small hole in the middle of the plastic to insert the funnel, tip down and duct tape it in trying to make it as air tight as possible. Pull a couple of strands of rope through the funnel. Leave enough length such that you will be able to spread the rope out the length of the area. This will make an air gap and allow air to pass.
Here's where you need to move fast: (2 sets of hands is VERY, VERY, VERY helpful)
Mix up pot of resin/epoxy
Wet out area of deck you are working with
Press up matte glass and wet out some more
Roll out evenly with laminating roller
Roll on resin/epoxy with paint roller (a good coat)
Press up core
Press up plastic against core while friend spreads out ropes and then duct tapes perimeter
Have your friend put shop vac hose on funnel and duct tape
Have friend continue to duct tape to seal all leaks which will ultimately cause enough suction to hold up everything, pull the resin through the core, and keep even pressure while everything kicks off
Leave vacuum running for about an hour or until satisfied with how well it kicked
Do not remove anything until hours later. I'd wait until the next day
3. Pull away all duct tape, plastic, etc.
Sand and fair
Fill any gaps with thickened epoxy (peanut butter again)
Let kick
4. Sand and fair
Wet out and lay out fiberglass cloth. I would recommend 2 layers
Roll out evenly with laminating roller
Let kick
Tips:
Keep deck shaded from the sun, If it gets hot the glass/core will not have a good initial tack (which makes this very not fun)
Don't put a light just underneath where you are working for the same reason
Keep lots of paper towels and acetone on hand
Clean laminating roller(s) and paint rollers thoroughly
If you need me to clarify any points let me know. It's a lot of steps. I did my best to be clear and concise.
Hope that helps!