Question on fiberglass repair

[Bay Tripper]

Hello,

I am still relatively new here, and I have done a lot of searching online and on this forum, but I am still wondering about one thing.
I am about to tackle replacing some rotten core in my transom. My 1988 H26.5 has a small outboard motor mount and there must've been water getting in through the mounting bolts. I have bought and read the West System book on fiberglass repairs and have all the materials in hand, but I'm not sure about one thing.
The repair will be done from the inside of the lazarette. It is a great place to do my first FG repair, as it doesn't have to look pretty. The inside of the lazarette is just fiberglass that looks to be painted. Especially the woven roving in the one corner. I seem to recall reading/hearing that you can't epoxy over polyester paint. Is this polyester paint, or is it epoxy paint, or is it just tinted epoxy? Do I need to sand off the "paint" in the areas that I will be laying up new fiberglass for the core replacement? Do I need to totally remove the paint, or just rough it up?
What kind of surface prep do I need to do to the areas that I will be epoxying onto? Any help would be greatly appreciated.

Thanks,

Ryan

 

[dlochner]

It is probably paint and you'll need to remove it. The inner layer of glass needs to be removed, the rotted wooden core removed and a new glass and resin laid up.

The new glass should bond directly to the old glass without an intermediary layer.

 

[31seahorse]

Hello Ryan,
Assumptions: The boat is out of the water; You are removing the motor and then the motor mount with its through bolts and its backing plates; You already know (or will discover) what core material is in the transom.

First, purchase and use the most protective dust filter you can find. Not just a pull-over your mouth and nose dust mask! Eye protection and a painter's suit too. Second, use a vacuum whenever you are grinding or cutting the inner layer of fiberglass. A Dremel vibrating tool or similar brand will work in that confined space and will create less dust than an angle grinder would throw. The flat wood/metal blade won't last very long, so have a few on hand, but you probably won't need to remove a huge amount of area, but don't be too timid. Mind how deep you are cutting......you do not want to cut through to the outside! The plywood or other core material will come out easier. You need to remove all the ruined core. If you need to remove core down to the curve of the hull let a curve of original fiberglass standing an inch or more high on the inside so you can "tab" the new fiberglass to it for a stronger joint. Third, I would put West System on the aft side of the replacement plywood to bond it to the outer hull layers you are leaving in place. Wipe all surfaces dust free with a thinner chemical and let it dry completely while you are outside away from the fumes. Ventilate the area too before going back to work. Fourth, perhaps, a single, larger backer plate would be a good idea. Work carefully and block the frame of the trailer (if you are on one) so the hull won't tip back with you working in the lazarette(sp?)!

 

[Barnacle Bill]

In your picture, that is not paint, but it is a polyester gel coat that is painted over the laminations of the transom. 31seahorse offered up some good points. If you have core material that is compromised from moisture absorption, you need to grind out the top layer of fiberglass (inside the lazerette) to get at the core to remove the bad, wet core. The best thing for you to do is to fiberglass the area where the outboard bracket attached leaving out the core and making it solid. The other areas you can put in new core material. This way, if you ever get water protruding via the bold holes, it won't be wrecking your new core. The moisture cannot go anywhere because it is solid. You need to use roving and alternating layers of matt gradually building it up to the thickness consistent with the rest of the transom. Also, as pointed out, you could add a backing plate on the inside to spread out the load and offer more strength. Remember when using epoxy, you must protect yourself. You can get senstitized to epoxy if you expose yourself to it too much. If that happens, you can't even be in the same room with it. So, wear a mask, gloves and protection as suggested above.

 

[Parsons]

Agree with @Barnacle Bill 's comments about making the area directly around the bolt holes solid, rather than coring with wood, to prevent future rot due to the inevitable leak around bolt holes. May I suggest that you consider some engineered material, such as G10 fiberglass board rather than trying to make your own solid transom in the bolt area? For about $100-200, you can embed G10 to cover that bolt area and save hours of individual lay-up. If you try to laminate more than a few sheets of glass mat at a time, you will invariably get bubbles (weak) and heat buildup due to the exothermic reaction of the epoxy. The reason for core is that it saves both weight and time in building.

 

[dlochner]

The entire damaged area does not need to be solid glass. The core is probably plywood and with a backing plate it should be adequate to support the motor. The new core should be bonded, i.e., epoxied to the outer skin as well as the good core surrounding it. The idea is to transmit any forces to the rest of the transom. If the new core is not bonded to the old core the transom could flex causing the skin to crack and you'll be back to square one.

The holes for the mounting bracket should be treated in same manner as any hole into a cored deck, drill oversized holes, remove a little coring around the hole, fill with thickened epoxy, and then redialed to the correct (smaller) size. Here's an article on how to do this. https://marinehowto.com/bed-it-tape/