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Replacing a Fiberglass Boat Floor

05/14/2010 12:55 PM

I have a boat that has sat out in the elements and has destroyed the boat's floor.

The runner's in the bottom along with the foam thats between the runner's is in great shape, there is no moisture and the wood runners look like brand new.

I want to re-fiberglass the floor , I have taken the old floor out but then realized that the floor was so bad that I am not sure if I need to put down a wood floor first then fiberglass over it or ???????

If I do need to put in wood first what thickness do I need ? The boat is only 16' but the floor is only 13' long by 8' wide plus I have two chairs that I need to re-bolt to the floor.

Thanks in advance!!

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Re: Replacing a Fiberglass Boat Floor

05/14/2010 2:32 PM

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.

The first 5 days after a weekend are always the hardest................................
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Re: Replacing a Fiberglass Boat Floor

05/14/2010 11:03 PM

Everything mentioned was perfect---( I Still build boats)---Remember though, Acetone, if used too heavily, can cause structural problems with the laminations.. As previously mentioned, just use it as a wipe down--If not, Lacquer thinner will suffice, as many people like it because it is not as volatile, and does not evaporate as quickly. On top of chemical bond removers, I always like to use mechanical "teeth". I.E. a rough grit sand ing of the to-be bonded areas--THis improves on the laminate bonding and strength, and removes some potential contamination....Use either epoxy or polyester resin , mixed with micro balloons as a fillet between the deck and hull side, and you are good to go! Keep us up to date! Mac

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Re: Replacing a Fiberglass Boat Floor

05/15/2010 2:19 AM

A friend of mine, running a fishing lodge, has rebuilt about 15 Bertram 31's, which originally had wooden decks and bulkheads (the older ones, at any rate). Having several very similar boats, he came up with a specific design for the various components. The floor panels (removable) were foam-cored fiberglass (alternating layers of mat and roving), laid on a table covered with waxed formica. The foam was laid on top of this (I'm not sure of the specs on the foam, but it was designed for this sort of work), then more layers of fiberglass. When cured, the panels were popped off the formica (the wax serves as a mold release), with a perfect surface, cut to shape, then coated with non-skid where appropriate. No wood. In the real old versions of the boats, the original stringers (wood) would be glassed over in place...

These boats see a lot of heavy work, and yet need minimal maintenance (painting about every two years- amazing what a 1000 pound marlin can do to one of these boats!)

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Re: Replacing a Fiberglass Boat Floor

05/15/2010 4:19 AM

Hi, This is a very common problem and here at Explorer Marine we have had to deal with it often. The comments have already covered most of the issues, however, I would add a few more thoughts. 1) Acetone is a solvent for polyester resin - not a thinners, so leaving acetone on any surface will degrade the composition of the resin. 2) We bond to marine ply by sanding with a very coarse - 36 grit - angle grinder immediately before coating with "hot" resin. That is with a very high catalyst percentage. If you lay resin, mat resin and roll immediately the bond will be good. 3) Remember - if you intend the boat to last a long time, to lay GRP on the bottom of the ply first, then cut to shape and lay some on the edges. We then sand and lay the top to bond the floors into the boat. 4) In order to ensure the strength of the ply to hull bond, use bias cut rovings along all sides, and at the joints between the ply panels. 5) For a good non slip flooring, we use "Treadmaster", which is excellent but expensive. Cheaper options are industrial flooring, or flowcoat textured with a roller or with coarse sand. If you want to have a look at what we build, visit Good luck

Hugh Mattos Chartered Engineer...... :---------: Through helping others we give purpose to our time on this earth and take pleasure from it.
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Re: Replacing a Fiberglass Boat Floor

05/16/2010 1:12 PM

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.

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Re: Replacing a Fiberglass Boat Floor

05/16/2010 11:11 PM

You are getting good advice. Before you start, you might benefit from taking a look at the US premier source for helping people with such projects.

Check for a wealth of information and case studies from folks like you. You can also try for a library of videos including some West System, all for the watching.

West also offers lots of HOW TO books on the field.

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