What kind of Fiberglassing compound to use?

[scottrbishop]

Hey everyone,

I am replacing the transom in a Glastron gt-150 fiberglass ski boat. I have all the wood out and I'll be spending my day cutting templates of the transom and floatation boxes.

I'm confused on what sort of giberglass to use to glass it back into the boat. I'm using 2 .75 inch sheets thick of Marine Plywood that I will be putting a fiberglass mat in between and glassing together. Should I use an epoxy resin, fiber glass resin, or vinyl esther to attach back into my boat? I know epoxy attaches to poly, not the other way around. Should I use epoxy just to be safe? I assume I'll have to use something different to repair the cosmetic gelcoat where I had to cut the 2 apart.

Any help is appreciated!

Thanks,
Mike

 

[KnottyBuoyz]

Re: What kind of Fiberglassing compound to use?

Hey Mike

Putting mat inbetween the plywood won't help much. Use a thicker or thickened epoxy to glue the two sheets together.

Epoxy is your best bet for that type of repair. When choosing fiberglass for a transom repair look for "biaxial" or "triaxial" fabrics. These have the fibers running straight and crossing each other at angles. The layers are stitched together to hold them. The glass fibers are stronger when kept straight so steer away from knitted or woven fabrics. You'll need weights of 12 oz or 18 oz biaxial. It can also be bought with mat attached which will help it bond to the plywood. These are normally found as 1208 and 1808 fabrics. The 08 is for the weight of the mat. You'll also need some fiberglass tape, I use biaxial tapes as well, much easier to handle and strong.

As for repairing gelcoat, well that's out of my area. I'd be looking at paint after a large repair like that. Plenty of threads on here about various paint products.

 

[scottrbishop]

Re: What kind of Fiberglassing compound to use?

With epoxy I can bond wood directly to fiberglass, correct? Same with bonding wood to wood? Do I need any interface layer of mat?

 

[Coors]

Re: What kind of Fiberglassing compound to use?

Some local boats stores carry MAS. Otherwise google them.

 

[KnottyBuoyz]

Re: What kind of Fiberglassing compound to use?

With epoxy I can bond wood directly to fiberglass, correct? Same with bonding wood to wood? Do I need any interface layer of mat?

Yes you can bond the wood to the fiberglass. Make yourself an epoxy glue of epoxy and wood flour or epoxy and milled fibers. Get it nice and thick like peanut butter and use that to bed the wood onto the fiberglass hull. Once you get it in there radius the inside so that when you lay on some fiberglass it'll have a nice smooth tansition from the wood to the fiberglass. Two layers of 6" glass tape overlapped about 3" should work nicely. Prep the fiberglass by sanding back (rough is better in this case) about 4" from where the wood will go, wipe it down with acetone to clean it up.

It should look something like this when you're done....

Transom on the right, old hull on the left. Nice smooth transition from transom to old fiberglass.

Here's a wider shot.

 

[pitahui]

Re: What kind of Fiberglassing compound to use?

While epoxy is easier to work with, a better adhesive, and mush more water resistant than polyester resin you cannot apply gel coat over it. If you do the repair with epoxy you will have to paint the transom. Epoxy is also susceptible to ultraviolet light (sun) damage. It has to be covered. If you use polyester resin, then you can gel coat the repair. Coat the gel coat with PVA before it dries which will give you a hard surface. The PVA (poly vinyl alcohol) is washed off after the gel coat dries.
Putting fiber glass mat between the two sheets of ply will make the transom more rigid (less prone to flex).

 

[scottrbishop]

Re: What kind of Fiberglassing compound to use?

While epoxy is easier to work with, a better adhesive, and mush more water resistant than polyester resin you cannot apply gel coat over it. If you do the repair with epoxy you will have to paint the transom. Epoxy is also susceptible to ultraviolet light (sun) damage. It has to be covered. If you use polyester resin, then you can gel coat the repair. Coat the gel coat with PVA before it dries which will give you a hard surface. The PVA (poly vinyl alcohol) is washed off after the gel coat dries.
Putting fiber glass mat between the two sheets of ply will make the transom more rigid (less prone to flex).

I have Bi-axial Fiberglass Cloth w/ Mat Backing... Could I use this between the two sheets of plywood to make more rigid?

 

[erikgreen]

Re: What kind of Fiberglassing compound to use?

It won't do much for rigidity. About all it would do is help adhesion on the side where the mat is.

Just use the appropriate thickness of wood for the transom and you'll be plenty rigid

 

[Coors]

Re: What kind of Fiberglassing compound to use?

And with the mat, it will be thicker than it was; Use epoxy with micro balloons-the balloons keep you from clamping it too tight.

 

[ondarvr]

Re: What kind of Fiberglassing compound to use?

Don't stress too much over whether you use epoxy or polyester, it won't really make a difference and you won't be able to tell the difference between the two in the future. They each need to be used correctly for good results though.

Using glass between the plywood won't do much to make it stiffer, it's the thickness of the core (in this case plywood) and the skin on each side of the core that makes it stiffer. The transom will have bolts through it from the motor and stern eyes that will be clamping it together, so coming apart isn't a big concern.