Encapsulating stringers

As I am about to make a decision on stringer material today (douglas fir / spruce ) for my 18 ft OB, I plan to encapsulate them prior to installation. I will be using polyester resin, my question is how to acheive maximum saturation of the resin into the spruce, I have heard that thinning the resin with acetone prior to the addition of the MEKP, but what should the ratio be prior to the addition of the catalyst?

Another quick one while I am here...If I am completely encapsulating the transom as well, can I get away with plywood other than marine grade? I am not trying to cheap out, just like to know the why's of it all. This boat is trailered, not moored.

-Ok J

 

Thinning resin is a mistake. It will make it weaker and it will absorb water. You can use exterior grade plywood for the transom.

 

any info on how to increase saturation?
Thanks.

 

Ok J,
Sure I can give you my opinion, epoxy will stick much better to both wood and old fiberglass, it'll also give you more time to work.

In general you want to make sure that all sharp edges are rounded off and all inside corners have a generous radius, (fillet) at least big enough for your hard-roller to fit into, I like to fill empty caulking-gun cartridges with thickened (non-sagging) epoxy, shoot a bead in the corner, then use a rounded-off mixing-stick to make the radius. Before you start laminating, make sure the glass you intend to use will go around all the corners without leaving air-bubbles, if not, use more layers of a lighter fabric.

For making epoxy resins penetrate, I prefer heat instead of thinners, for small pieces you can make a temporary oven from a cardboard box and adjust the heat by moving a space-heater back or forth from an opening in it. (a couple of digital cooking-thermometers is a good investment) For bigger things you can build a tent or other temporary enclosure. Once the substrate is up to temperature, mix a small batch of resin, microwave it for a second or two per ounce, then put on a thin coat (thin coats penetrate better because the air trapped in the surface porosity has to escape through it) very quickly.

I don't think thinning is really neccessary in most cases when you use epoxy though, and using space-heaters as I outlined is dangerous and cannot be left unattended.

Also, see the thread called "chainplate repair" or something like that from a couple of weeks ago, it's got some additional applicable information.

Yokebutt.

 

The only time I ever had a concern over penatration was when I had to do a full end grain glue job. They should never have to be strong. There is very little penatration on any non- end grained surfaces. Do 10 practice glue ups. Beats talk. A fresh lighty sanded surface of " difficult " wood types removes the surface oil or liquid. Gule as soon as possible. Other than that, you are Certified.

 

stringers

Seems to me you'd be 100% better off to make your stringers out of glass, perhaps using polyurethane foam as a form and get rid of the wood...

 

To continue on Lorsails line of thought, you could slap together a cheapo mold from melamine coated particle-board and make hollow fiberglass U-sections. If you start with a layer of peel-ply on the outside you wouldn't have to grind away the mold-release. Any work that can be done on the bench instead of in the boat is usually a lot easier.

Yokebutt.

 

I take a can and poke the corner of a plastic baggie down into it and drape the rest over the edges, sort of like lining a garbage can with a trash bag. With a putty knife I put the filler in there, then take it out and twist it up and snip the corner off with scissors so I end up with a disposable miniature cookie dough bag squirter kind of thing. After putting a bead in the corner I finish the radius with a socket of the appropriate size from the toolbox. I like to use my empty caulking cartridges in the potato gun. Sam

 

Quite right, SamSam, I've done the same thing with ziploc bags, it's amazing how resourceful you can get after the hardware store has closed for the night.

Yokebutt.

 

One note on penetration. Heating the wood helps to draw the resin in, be it poly or epoxy. This is a matter of 100ths of an inch we are talking about here though.

Steve

 

Hey, where do you find those empty caulking gun cartridges???

Steve

 

West System has them. I haven't tried a hardware store.

Phil

 

Thanks...
Steve

 

Lewisboats,
I just gave him an answer because he asked the question. Do I think it's a major concern? No.

Yokebutt.

 

OK j
If your worried about saturation why not give the stringer an anchor pattern. Use a bed of nails or a coarse paper with a cross hatch pattern. I prefer epoxy. Overcoat and build up times are on your side. I haven't used poly. for a long time and can't remember what your time line is like once you promote it. I like the idea of warming the epoxy. Not sure how it affects your working time once you add the hardener or if you would use a slower hardener? Perhaps someone could address these details.
Good Luck
Phil