T&T: Drilling fiberglass

[Albin43SDtr]

'Lo All,

I have successfully drilled and tapped fiberglass for mounting 
various things on my and other folk's boats. None have ever failed 
that I am aware of. The general methods I used are:

If sufficient thickness of fiberglass is available, I use pilot drill 
and a chamfering bit, as suggested before, to prevent gelcoat 
cracking. Then drill to the proper size and tap the hole. I usually 
use 5200 to seal and secure the machine screws. Self-tapping screws 
tend to fracture the  resin, unless the hole is too large, which then 
lets the screw "work", even when a sealant/adhesive is used.

If there is insufficient thickness of fiberglass, I use a backing 
plate (sometimes SS or aluminum, sometimes made of fiberglass), 
usually epoxied (mixed as a chopped fiber slurry) to the underside if 
possible. If not, I use a mounting plate on the outside that has a 
larger diameter and greater number of machine screws, installed as 
above. The base of whatever I am mounting is mounted onto the plate, 
again using machine screws.

If the fiberglass is cored, I have used the "Allen wrench" method to 
remove the coring around the holes. I usually used a drill one size 
smaller than that required for the tap, so that when the hole is 
re-drilled, the threads will be in "virgin" fiberglass. The arm of 
the wrench is long enough to reach over half way to adjacent holes. 
Then I vacuumed the debris from the hole(s), filled a caulking gun 
tube (a syringe would probably do the same thing) with a fairly thick 
slurry of microfibers and epoxy - a little more than sufficient to 
completely fill the area cleaned out. Chopped fibers (the ones I 
used, anyway) do not work using this method as they tend to clog the 
caulking gun. Using a caulking gun gives sufficient pressure to force 
the microfiber slurry into the entire cavity, until it squishes out 
of all the holes (this is the reason for making sure the holes 
"connect"). Use a small stick and/or the Allen wrench to work the 
slurry into all the surrounding edges of the hole and to remove any 
trapped air, then use the caulking gun again to assure that all voids 
are filled as much as possible. Let fully cure, then re-drill the 
holes, tap, blow debris out of the holes with compressed air (don't 
get it in your eyes!!!!), insert sealant (I usually use a Q-tip to 
smear the 5200 thoroughly onto the threaded fiberglass, and into the 
threads on the machine screws). I must admit that I can only recall a 
couple of times that I had to use this method. It is quite labor 
intensive and tends to be quite messy, thus requiring surrounding 
areas to be protected with a couple of layers of taped-down plastic 
or heavy paper. When using this method, be sure to have sufficient 
working time for the epoxy and an ample supply of acetone or MEK and 
rags for cleanup. Wear protective gloves.

Before the advent of the various epoxy formulations, I used polyester 
resin. I always had good results with it, too. In fact, I still 
prefer to use polyester resin whenever it is suitable. While I was 
never in the boat repair business, I maintained a small fleet of dive 
boats, helped friends with theirs as well as built 
(one)/modified/maintained my own boats.

Therefore, I do not claim "professional" training and experience. The 
above is what worked for me. YMMV


Take care and be safe.

Wayne
Celestial
Albin 43 Sundeck
Near Panama City, FL