Replacing a Fiberglass Boat Floor

Ahoy there fellow boater,

This is probably more info than you wanted but here goes -

My wife and I owned a wood, 28' - 1968 Chris Craft Constellation for 18 years

When we bought the boat the decks (floors - wood with marine vinyl glued down) were rotted and had to be replaced.

Here is what I did:

I removed all of the decking and support bracing underneath. I installed new bracing and screwed down treated ¾" plywood. Be sure to apply a few coats of fiberglass resin to the sides and underside (2" perimeter all the way around) of the plywood after cutting and before screwing down in place.

I scuffed off the gloss (from pressure treatment) from the plywood and wiped down the plywood with liberal amounts of Acetone.

I then stapled down (brass staples) fiberglass mat. I used mat rather than mesh as I was told this would add strength due to the non-uniform pattern of the mat.

Once the mat was tacked down I mixed fiberglass resin. As I applied the resin, I rolled it out with a fiber glass roller.

In case you are unaware, a fiberglass roller looks like a paint roller except instead of having a cloth roller, it has washers loosely mounted where the cloth roller normally goes.

When you are mixing your resin, start out mixing small batches at first because it tends to set up fast especially if it is over 70 degrees out (avoid working with resin in direct sunlight). Once you get a feel for the set-up time you can increase your batch size.

Pour the resin over the mat and roll in crisscrossing directions. Be sure to roll out all air pockets or the cured fiberglass will lift from the plywood (you will be able to see air pockets moving around as you are rolling).

Avoid over rolling the mat as you will distort the distribution of fiber which will give you an un-uniform surface.

Once the resin is cured (I always waited 2 to 3 days), sand the surface to your liking (wear a respirator at all times while sanding). Don't sand into the mat.

If there are deep imperfections, mix a batch of Bondo and add a small amount of mixed fiberglass resin (about 2 oz resin to each 1 lb of Bondo) to fill in deep imperfections. Bondo by it's self will absorb water over time but when you add resin, the Bondo becomes pretty impervious to water.

Once your satisfied with the texture of the surface, wipe down with Acetone to remove sanding debris and apply gel coat. The gel coat can be brushed or sprayed, I brushed it on and it looked perfect, like it was sprayed on.

For a non-slip surface mix non-slip sand additive (sorry, don't remember the trade name) which you can purchase where you buy your resin.

Be sure to caulk thoroughly between the inner gunnels and the deck otherwise your new deck will begin to rot along the edges sooner than later.

You can still put carpet down if you like but in either case I would gel coat the resin to seal it.

The repair I did lasted the 18 years we owned the boat.

As far as the size of the area you are working with, I think 1/2" to 5/8" plywood will suffice.

Good luck and happy boating/fishing.

Have fun with this one. Its always great to see an old girl coming up all smart again. You did not mention what type of boat it is. On an inland boat I would go with 1/4" marine-ply with 18oz glass on both sides. Build in re-enforcing for seat mounts.

Stay away from Acetone..... Use Styrene monomer to activate the top layer of wood to get a good bond with the resin. Wet the wood quite well with it. Degrease old glass with a strong degreaser, sand with 60 grit and then wet with styrene monomer just before you lay up new glass.

If you are using pedistal seats add another 1/4" wood/ glass layer underneath the area the pedistal is to be mounted. then fix an anchor plate both inside and out for the pedistal.

If it is an offshore boat double up to 1/2" marine plywood with the glass on both sides.

Speak to your resin supplier, they will have a heavily filled, "bondo" like Bonding-resin you can use to bond the new floor to the runners and the sides. Do test runs for set time, and then with 2hour set time get a helper to mix about a pound at a time, scoop it into a heavy plastic bags and cut a corner off, pipe it onto the runners in thick beads using the bag like a confectioner's icing bag. Then drop your new floor in and screw it down. Have two dozen 25lb sandbags ready to weight the new boards down while you run the brass/stainless steel screws in with an electric screwdriver. Dip all the screws in melted beeswax before you start. It helps to run them in and also seals the hole to prevent water seeping an rotting the wood. (Old school, but it works)

Where to put screws..... When the new boards are glassed and fitting nicely, lift them and place little balls of coloured moddeling clay on the runners where you want your screws, refit the boards and presto when you now lift them you have drill marks to to drill from the bottom of the boards.

Remember to use a "Waxed Resin" so that it waterproofs well. Only drag with it is you must sand well before you lay the next layer or bond it or the wax will release the new layer. A layer of flow-coat is important to finish both inside and out to prevent water seepage. Apply the top coats of flow-coat with a brush, sand flat and then second coat with a squeegy to level it off.

Finish the deck with short open-pile neoprene backed carpet in a cool colour, not too dark of it toasts your feet.

Good luck, please keep us posted.