: . . .Another thought I had was vacuum-bagging the hull when 'glassing/epoxying it.
: Has anyone done this? Is it possible and how would/did you set it up?

First take a look at Ray Jardine's webpages where he shows how he did a vacuum- bagged kayak. His home page is impressive at http://www.rayjardine.com/ But if you are in a hurry, just jump directly to his kayak building log:
http://www.rayjardine.com/adventures/kayak-construction/index.htm

A few years ago he had a slightly larger description, but he has edited it down to a mere 36 pages, and I think he removed too much technical info--but it still shows the methods and discusses the materials.

: My thought was to lay the glass down, epoxy, then cover with plastic sheeting
: that was taped to the strongback. This should squeeze out any excess epoxy
: and let it drop down to the strongback so it won't pool on the boat.

If your epoxy is thin enough, and takes long enough to gel, you won't need the plastic. Almost all of the resin will simply drip off the boat by itself. It won't pool on the boat, it will pool on your floor. For most people that is not s a desireable outcome. That is one reason why there is such a variety of resins with different viscosities and pot-life times. Those which we currently use for boatbuilding are thick enough to stay in place for the hour or so it takes to put on the glass and wet it out. Then, if you've picked a good mix of fast or slow hardener, which agrees with the working temperature, the stuff will gel, and not flow any further as it polymerizes into a solid plastic. With vacuum bagging the time involved could be different that hat you are used to with hand applied resin, so do a smaller practice run to see how long it will take--and plan on mixing your resin (or even buying different resin!) so you get a material which works in the time you will have.

For a rough guess for a first timer, I'd suggest a slower setting resin. That would give you time to get the stuff placed before it gelled. The bag will keep it from running out, so you'll just need to wait a bit longer for it to cure. Actually, this should be a shorter time opverall than working by hand, as you'll be putting on a coating which is thick enough to fill the weave on your first shot. That thicker coating of resin generates a bit more heat than a thin coat, and would set up a bit faster than a hand-appplied coating anyhow.

In an ideal world there would be no intermediate sanding, washing to remove blush, or fill coats. In our reality, you'll do some light sanding to remove blemishes left from wrinkles in the bag, and probably do some touchups (partial fill coats) where the cloth was starved of resin and the weave shows.

But we are getting a bit ahead of ourselves. Let's step back to your idea of taping down some plastic over your wet glass fabric. The tape would indeed put some pressure on the fiberglass and resin. But not very much. In fact, compared to vacuum pressure, the effect would be paltry. look at the numbers. Atmospheric air pressure at sea level is about 14.7 pounds per square inch. (that is about 6.66 kg for an area 25.4 x 25.4mm, which is about 6.45 square cm. Rounding off, you've got about a kilogram of air weight (or pressure) per square centimeter. A common building brick weighs about 2.25 kilogram (about 5 pounds), but it has an area of about 4 x 8 inches on its biggest face. (roughly 10 by 20 cm for US bricks, or 215 mm by 110 mm for UK bricks) So that weight is spread over a 200+ square cm area, or 32 square inches.

Now, I don't know how strong you are, or high tightly you can pull that tape--but I'd bet that you would get more pressure applied to your fiberglass by putting a brick on it than by trying to tape it down. But that is still trivial. To get anywhere near the pressure you could (theoretically) apply by using a vacuum bag you would need to apply a stack of about 88 bricks, which would be about 18.5 feet, or 5.7 meters, tall.(exclusive of any mortar to hold those bricks together.) But that is just theory. In practice is is fairly easy to get 1/10th that vacuum--or the equivalent of a half-meter-tall stack of bricks over every inch of the surface of your lightweight kayak!

You have to keep this incredible potential pressure of vacuum bagging in mind throughout your building process. Should you try to slip a bag over your entire boat and strong back, for example, the bag will not be supported very well as it passes over the building forms. As the vacuum increases the bag will rupture. If it didn't, then the presure would cause the strips to bend and when the resin had hardened, your hull would be deformed.

To avoid this, you'll have to remove the hull from the building forms before bagging and glassing it. This is not something you are going to look forward to, particularly if you are building a very lightweight boat. The strips are simply edge glued and all those joints are fragile. Any glue which leaks through and glues a strip to a building frame is going to have to be broken free--but with great care. Once you finally get the wood core off the building froms it is going to be easily deformed by its own weight. Adding glass and resin to it will increase that problem--so once the thin is off the building form you have to slip it in a vacuum bag, and then get it back on the building forms so it goes back into its proper shape.

OR, you can plan on building with the vacuum bag as part of the stripping process! Here is one way to do that. Start by cutting your forms 1/4 inch (6mm) smaller than the design calls for, set them up and align them normally, then roughly and quickly cover the forms with 1/4 inch or 6mm strips which are spaced 1/8" to 3/8" apart(3mm to 9mm). These strips should be made from an inexpensive common wood. This becomes a building "bed" for your boat.

Over this you lay the plastic for the inside of your vacuum bag, and smooth it to fit. Remove this plastic and set it aside. Put on a lyaer of cheap polyethylene plastic (disposable drop cloth stuff) This will serve as a barrier to prevent glue drips from causing problems. Next, do your stripping over the plastic covered bed.

I'd use staples for a project like this. You can drive those staples alomst anywhere as they'll either hit a building form, or one of the underlying wood strips. In fact, I'd avoid aiming for a building form and just use the staples in random and infrequent spots. Such random holes are not going to be as obvious as a row of staple holes going around a boat where the building form was. I would use an epoxy glue for gluing my strips together. I would mix small quantities and apply the stuff sparingly and with great care to avoid drips. A fast setting epoxy doesn;t need tight clamping, won't run very far before it gels, and it won't adhere to the underlying polyethylene plastic.

Once the boat is stripped I'd pull most of the staples and fair most of the hull, leaving a few staples at each end to keep the boat in place. Once I had most of the hull done, I'd pull the staples at one end, fair and sand that, then finally pull the staples at the other end, use clamps to hold the hull to the building bed, and finish the fairing and sanding. T

Now put on your seal coat of resin, using a squeegee to barely coat the wook. The less you use here the better. Wipe it over the surface of the wood, and immediately after, wipe it off. What small amount remains on the wood will soak in and do your sealing. When it has hardened, do a light hand sanding to remove any roughness.

After unclamping the hull it should slide free esily, with that underlayer of polyethylene acting like a dry lubricant. If any glue or seal coat of epoxy has managed to seep between the strips during construction it will have a weak hold on the polyethylene. You could break it free now by bending the plastic sheet backwards and tugging--but why do it now? For the moment that polyethylene sheet is giving your strip a small measure of additional support. If you haven't already done so, trim it to within an inch or 2 of the hull strips.

Once the hull is removed from the building bed, quickly slide that premade bottom half of your vacuum bag onto the bed, then get your hull back on it. At this point prepare the material for the top of your vacuum bag. Cut it to size, and shape it as needed to suit the curves. Then remove it and set it aside.

Now you can put your glass and resin on, seal the bag, suck the air out and let the atmospheric pressure go to work for you. it is only at this stage where you need to delve into the mechanics of vacuum bagging. That is a whole other postiing (or more) and there are some really good videos and webpages which already go into this.

Do the hull outside first, then you can do the inside with vacuum if you wish. Once the outside galss and resin has hardened your boat should be durable enough to handle for the interior glassing without the additional support of that building bed. Keep track of how much resin you used for the outside. You'll use the same amount on the inside. Since the inside does not need to be perfectly smooth, many people use hand coated fabric, and don't fill the weave. That means less resin, and less weight. Also lower costs for the dispsables (tubing, breather/bleeder ply, peel ply, etc.) You might save some set-up time, too.

: Any thoughts? I considered it as a friend has a new carbon-fibre kayak and
: when ordered, he could have gotten 2 - 3 kilograms (4-6lbs) weight
: reduction if they bagged it (but added $1000 to the cost!).

Losing weight costs money. If you don't believe that, consider how much is spent on dieting each year

Good luck with your project. If you can find somewhere to rent or borrow an industrial strength vacuum pump you'll be happy. A vacuum cleaner will burn out.

PGJ