The lesser of two evils

I'm a newbie building a little plywood dinghy to play around in. I've been here long enough to know everyone reccomdends epoxy over polyester, but for someone with a tight budget like me, polyester is sooo tempting to use. But, I might be able to afford the epoxy if I stretch it thin with something like alcohol (I know, less than ideal) but that may or may not be better than simply using polyester. So, which is the lesser of the two evils for sealing plywood: using epoxy thinned out to the maximum or normal polyester resin?

 

Sealing a plywood dinghy? I'm not a boat person... how much are you gonna need? Maybe a gallon or two? And, correct me if I'm wrong, but since it's just being used as a sealer, it isn't structural in any way.

Here's an epoxy for non-structural aircraft parts. I've used it to make car parts and it works pretty well... it's a little bit on the thick side, so it takes extra care to wet it out, and the finished product is a little more flexible than, say, West System, but it's not nearly as flexible as you might imagine. It makes good stiff parts that'll flex a little bit more than with a structural resin- I'd bet that most people wouldn't know the difference (unless you used it to build a spar or something). As for the thickness, it is intended for laminating so it's not too thick, but it's not nearly as easy to wet out as West. I'd rather use West any day, but the price of this is nice. I use it when the part is non-load-bearing and I absolutely cannot overshoot a price point. $65/gallon.

http://www.aircraftspruce.com/catalog/cmpages/alphapoxy.php


But wait to get some more old-timers' advice before you rush out and buy the stuff. I don't want to be leading you astray. Personally, I've never heard or "stretching" epoxy with alcohol. It sounds like a bad idea to me... because I'm not a chemist and don't know enough to be comfortable taking that chance.

 

If you are not looking for a family heirloom go with the poly.
If repairs need to be made you can use epoxy later.
From my experience, poly doesn’t stick well to epoxy, but epoxy will stick to cured poly OK.

 

Epoxy is the choice, by far of the two, however it can be very unsatisfactory to stretch it out with solvents. I think alcohol is not really a suitable solvent anyway. What happens when the solvent evaporates away? The wood has porosity as the resin has not fully sealed all the fibres in the wood. Porosity equals ingress of water eventually. That equals rapid deterioration. Better to use a different system and follow the manufacturer instructions properly.

 

You would use acetone to stretch it out but it isn’t the best idea. Use the poly for now. Hatteras uses poly to make their multi million dollar boats (yachts).
Good luck
Ben

 

For making your own penetrating epoxy-
there has been a bit of conversation on this thread

http://www.boatdesign.net/forums/showthread.php?t=12061&highlight=CPES

Though it sort of paints a gloomy picture-
I am sure that it can be done.
IMHO, sevenup's suggestion of using laquer thinner seems to be the best "off the shelf product" to thin epoxy (other than epoxy thinner if available). Laquer thinner seems to have a reasonably close blend to the solvents used in CPES. Acetone by itself is most probably no good, as it has been advised that it will degrade the epoxy. Metho- definately not.

It should be clear that straight epoxy resin will do a better job than Poly resin.
However, thinned epoxy is an entirely different thing.
Rather than curing like a glass, it cures like a rubber. And as has been said, this rubber has little holes in it- which isn't really a problem if coated in a decent paint (exterior house enamel will do fine). Thinned epoxy is good for penetrating into and treating wood, as an undercoat for either paint, or for un-thinned epoxy- but will not act as a normal resin coating (epoxy or poly).
I guess the advantage of it is, that it improves the quality of the outside layer of wood. While if exposed it could allow water in very slowly (much slower than raw wood)- if covered it really should not- especially on a dingy that will probably spend most of its time out of the water. As it is throughout the wood, it cannot delaminate (without the wood!) and will help to protect from water invasion through scratches and chips- giving you more time to get on top of them...

Which is better?
a matter of opinion. And of how you use and treat it.
Some people think that penetrating epoxy is rubbish.
I don't.
I think it has its uses...
IMHO
I think that if you get some epoxy resin, and use it to glue all your joins (stich and glue is it?) and then thin it down and treat all of the wood, with at least a couple of coats (sanded dry in between or applied wet on wet)- then you would be in a better and lighter position than covering the whole thing in poly resin.

This is a contentious subject
and no doubt many will disagree.

If you cover it in poly (or epoxy), it will feel like a fiberglass boat.
If you cover it in penetrating epoxy and paint, it will feel like a wooden boat.

Good luck.

Hans.

 

Ditto on that, that Hans has indicated. Jeff.

 

I'm in the penetrating epoxy is rubbish camp. If you want to wet out well use a thin laminating epoxy at the right temperature (warm!). You will still need to go with several coats as the wood will absorb so much.