Thru Hull Fittings: Overdrilled, wood treated, filled with epoxy and redrilled

Once my transom repair was completed, I over size drilled out the critical load bearing holes for the motor mounts and tow hooks, treated the raw wood inside with penetrating epoxy, and filled the holes with thickened epoxy.  When it was time to install the motor and tow hooks, I had nice hard epoxy to drill thru with no exposed wood.  Here's some pictures: sorry for the layout, my editing skills need improvement.

Rot Doctor Clear Penetrating Epoxy Sealant (CPES), epoxy clean up solvent, West System Resin and Hardener, West System thickeners and fillers (structural), injector tubes, mixing cups, Personal Protective Equipment. (Always wear eye protection, one drop or splash = blindness!!!)

The more critical the application, the more important it is to do tests to get it right.  I wanted to make sure my mix was thick enough (ratios correct and written down), and I needed to know how much working time I had to fill the holes and work the product before it started to cure.

Before mixing my batches of epoxy filler, I set up my work site with my knee pads so I could drop right into them and get to work while the product was active and curing.  I also had clean up solvent, towels to rest the tubes on and to clean up over flow etc.

I ultimately discovered that two of these tubes would fill each hole.  So I mixed batches big enough for 4 tubes to fill 2 holes at a time.  I could have mixed bigger batches and used bigger tubes, but the product would cure faster than I wanted to be able to work the product.  See why below.

Nice fresh wood inside ready for CPES treatement.

I started each hole with a small pilot hole using a drill guide (Harbor Frieght has them cheap) to make sure I was drilling square to and thru the transom.  Before I progressed to the next larger drill bit, I countersunk the hole with a size larger counter sink, to avoid tearing up the gel coat.  I didn't have a countersink large enough for the final drill bit, but it didn't matter, see why below.  Warning Note: A lot of power is needed to drill thru 2" of wood and gelcoat.  Its only possible to do this by hand by progressing with larger and larger bits.  Don't go too fast or too slow.  With all the friction created and high drill speeds, if and when the bit jams, it can literally break your wrist.

I didn't care about tearing up a little gel coat with the final drill bit, because I created a flange on each side of the hole for better bond to the hull, and more reinforcement for the load that will be put on these holes.

The method of using clear velum and duct tape to hold the epoxy in the hole while it cured helped me see on both sides of the hole if it was totally full, had air pockets, and allowed me to work the small overflow into a nice flange on each side.  I used a small dowel to poke into the hole as I was filling to jiggle out any air bubbles.  Tape the bottom edge of the velum in place and let it hang down blow the hole(s).  Fill from the bottom back to the front top.  As the epoxy flows out of the hole work the velum upward to hold it in.  Then tape off the whole piece and work out the flanges.

An additional VERY HELPFUL TIP:  Leave pencil cross hair lines extended out from each hole.  They should have been there from the original drilling process to identify center.  Once the holes are filled and flanged, you'll need those lines to identify center again.