I’m going to assume, for the purposes of brevity, that we’re working with a finished, female mold, which is coated and sealed to the point of being airtight. It is easy to vacuum bag over a male mold, as well; the technique is identical. I’ll also assume that the mold has a peripheral flange (a sealed surface, not part of the functional mold surface, running the perimeter of the mold). If you’re working on a mold with no flange, vacuum bagging is possible, but you will still need an area, continuous with the mold-laminating surface, to seal to. If your mold is not airtight, I’ll make a note about that at the end. I’m also going to avoid discussing mold preparation techniques such as release agents, surface coats (gel coats), and general wet laminating techniques, although these skills will be necessary to arrive at a satisfactory result. Finally, I’ll assume the use of industry standard vacuum bagging materials and vacuum valve. If you don’t have a vac valve, you can skip it, see the note below. If you are not using purpose made bagging materials for this project, I’d really caution you to reconsider – they’ll make your life much easier.
Vacuum Bagging Materials
Vacuum bagging can be done over any combination of fabrics, resins, and cores, with a few comments – if using a core; it must be either contour cut or drilled through to allow resin and air to pass through. Resins must have a long enough working time to allow the lamination to be completed and vacuum bag attached before gelling. This may be hard to do with polyesters. Vinylesters will need to be promoted accordingly and epoxies will need a sufficiently slow hardener. If the resin system cannot be made sufficiently slow, the vacuum bagging can be done in stages, but this will require much more time and material. I’ll make a note on that later.
I’d suggest cutting the various bagging materials ahead of time.
Before laminating, sealant tape should be applied to the mold flange. Sealant tape is a rubber mastic tape supplied on rolls with a paper backer. It is applied ahead of time since it will not stick to a mold surface wet with resin. Follow around the entire mold surface with tape, making sure to overlap at the corners. It needs to be continuous. Leave the paper backing on for now. Incidentally, if you are using a semi-permanent type mold release, you’ll need to mask the sealant tape area before applying. Semi permanents have too high a slip to allow the sealant tape to stick properly after the fact.
Start Laminating the Canoe
The laminating technique should proceed as normal, ideally with less than 60% resin content. Most people have a tendency to over wet the fabric in an attempt to see it “wet out” faster. If in doubt about how much resin to use, proceed one layer at a time and use a scale to calculate the approximate resin required. Weigh the fabric (or calculate it’s weight based on area) for each layer, and mix an equal weight in resin, plus ~20%, which will give you just under a 60-40 resin-fiber ratio. This should be more than enough, including waste left in buckets and brushes. The exact amount needed will vary by the relative densities of the various fabric and resin combinations, but this’ll get you close.
While laminating, pay particular attention to avoid fabric bridging (fabric stretched in open space across a radius) or resin pooling. These are problems with any laminate, and vacuum bagging will not correct these problems.
Applying the Vacuum Bagging Materials
Once laminating is complete, vacuum bagging materials need to be applied in the following sequence:
- Peel ply is a fine fabric, polyester or nylon, laid against the wet laminate surface. Its job is to not stick to the cured laminate, and to impart a fine surface texture to facilitate secondary bonding on the cured laminate. It is available in two forms – coated and non-coated. Coated peel plies employ a silicone treatment to assist with release upon removal. Theoretically, the silicone can transfer to the laminated part, but this is not likely at room temperature, and not much of a concern for the average canoe builder. Most any peel ply will work; the release coated should be easier to deal with. Peel ply must be laid over the entire surface. It need not be continuous, but pieces must overlap at least a few inches. The smoother it goes down, the smoother your final inner canoe surface. Be sure it lays flat against the laminate in all areas, as it will not stretch, even under vacuum. It must be laid all the way the edge of the laminate.
- Next goes a perforated release film. This is a special plastic film punched full of evenly spaced small holes. While not technically necessary (and not included in a lot of vacuum bagging guides), I feel it is very useful. Its purpose is to control resin loss. I’ll explain that more in the next step. It has the secondary benefit of allowing easier release of the entire vacuum bagging material stack on cure. The small holes allow air to pass, along with a finite amount of resin. Again, this layer must lay flat, following the peel ply as closely as possible.
- Next is the breather/bleeder layer. This is a non-woven polyester felt type material, similar to quilt batting. It has two jobs – to allow even airflow under the vacuum bag, thereby assuring even evacuation of the bag and volatiles from the resin, and excess resin absorption. This absorption is controlled by the release film layer, for a couple of reasons – the breather is capable of wicking too much resin from an ideal laminate, causing dry spots, and a completely saturated breather layer will not allow air or volatiles to pass, negating the effects of the vacuum in that area. This is why I cautioned against excess resin – the release film will prevent it all from escaping (but this is better than the alternative of a dry laminate). Breather will stretch a bit, so laying it is a little easier.
- After the breather layer (we’re almost done), the vacuum bag itself. Vacuum bagging films are usually nylons, 1.5 – 2 thousandths thick. Although other plastic films can work, I don’t recommend them, for instance, vapor barrier would have to be more than 6 thousandths thick to be as airtight, and, at this thickness would be stiff and difficult to work with. Either way, bagging film is the least expensive of the materials used.
Standard bagging film comes in a 60″ width off the roll. Other widths are available, but 60″ is enough for most canoes. If you are doing a boat larger than this, make sure the film width is at least greater than the surface width at the point of maximum beam. Vacuum bag must fit the contours of the mold completely, but comes in a flat sheet, and must be continuous for an airtight seal. It is made to fit through a series of pleats – sealed folds in the film that take up the excess film in corners. For a canoe shape, it is sufficient to make two pleats – one at each end, the full width of the film. (Note that stretch films exist which allow the use of fewer/smaller pleats, but these are fragile films, easily punctured, and a canoe shape will probably still require a pleat – they do not stretch that much.)
Cut the film to the full length of the mold. Start with one corner at the apex of the mold. Peel off a few feet of sealant tape backer, stick the corner of the film at the apex, and proceed to lay the edge of the film down on the sealant tape, as smoothly and with as few wrinkles as possible, around the canoe perimeter. Wrinkles are leak points, so try to avoid them. The film will only stick lightly to the tape if not pressed, so it may be pulled off with a sharp tug if you go awry. One seated smoothly and evenly, press down along the tape, pull off a few more feet of tape backer, and continue until you get to the opposite apex of the mold. Stop, go back to the original end, take the other corner of the film, begin at the original starting point, and cover the other side of the mold. At this point the whole flange should be sealed to the bag, with two large, open folds at the ends, excess bag due to the tapered shape of the mold. These folds will be the pleats.
- Now the tricky part (to explain, anyway). Cut a piece of sealant tape slightly longer than the length of a pleat (in this case, about 32″, half of the 60″ film width plus a couple of inches). Beginning at one apex of the mold flange, attach one end of the free sealant tape, backer attached, to the sealant tape on the flange, where the two corners of the film meet. Hold the pleat taught (or better yet, have a friend do it), while you apply the new tape along the inner surface of one edge of the fold. Like the flange, you don’t want any wrinkles in the bag. Follow along until you get just past the top of the pleat. Seat it to the film using your thumb and forefinger. Now, starting at the bottom near the flange, remove a few inches of tape backer and press the second side of the pleat to the first. Again, no wrinkles. Follow along slowly until you get to the end of the pleat. Pull out the rest of the backer, and if you’ve done it carefully, the end of the sealant tape should fold over on itself in the corner making a perfect seal. (Note that this, being the trickiest part, could be practiced on a flat surface like a tabletop well ahead of laminating. In fact, I encourage this heartily.)
- Now the opposite end. It goes the same with one exception. Cut a piece of breather a little more than the length of the pleat, about 4″ wide. Cut an equal, length of release film, about 6″ wide. Place the release film inside the bag, leading from the breather on the mold surface to the apex of the pleat. Lay the strip of breather on top. Place the plate half of a vacuum valve on the outboard end of this breather strip. If you have no vac valve, do worry, skip it, and just make sure the breather does not fall back onto the canoe surface.
- Repeat taping and sealing the pleat, as done at the other end of the boat.
- At the vac valve plate, (very) carefully cut a slit at the top, and insert the mating half of the valve. If you are not using a valve, see a note below. Make sure all materials are still in place and start your vacuum. Depending on your pump, it may take a while to draw a full vacuum. I’ll make a note about that in a minute.
- Inspect the entire sealed perimeter, looking for wrinkles and folds in the film at the tape surface. Apply more sealant as needed to seal these. If you listen carefully, you should hear any leaks. Pay particular attention to the pleats and corners. Seal as necessary.
- If all goes well, you should see the bag draw tight against the mold. Make sure that the bag is drawn tight against the full mold surface, particularly in the fine ends of the canoe. If the bag does not draw down tight it is either leaking, or is bridged. Leaks must be dealt with first. Bridging can be solved be removing the vacuum source until the bag relaxes and carefully repositioning the bag until there is enough slack to fit the contour. Reapply vacuum and re-check. Again, the fine ends of the boat will likely be the problem areas.
- Leave the vacuum source running until the resin cures.
Notes About Vacuum Bagging a Canoe:
A vacuum bag must be attached to an airtight surface. If your mold is not airtight (ie, mdf or strip plank) a vacuum bag can be used around the whole mold, in a technique called envelope bagging. This involves laying a vac bag (film only) on the backside of the mold, and sealing the top bag to it. Since canoe molds are large and unwieldy, this would be very awkward, and the outer bag easily damaged. I don’t recommend this; I only mention it because it is possible.
A vac valve is a two-piece coupler device used to apply vacuum through a bag. A metal plate goes on the inside, a hole is cut on top, and the top half attaches with an airtight gasket, usually with a half twist. If you do not have a vac valve, a vac hose can be inserted through the bag as such: on the bag at the entry location, apply a circle of sealant tape with an inside diameter equal to the O.D. of the vac hose. Carefully cut a hole through the bag in the center. Apply a loop of sealant tape around the circumference of the vac line, a few inches from the end. Push the hose through the bag until the tape on the hose and the bag meet. Squeeze tape together until sealed. Note that, inside the bag, the hose must end on the piece of breather in the pleat that I referred to in the previous post.
If a fast cure resin system is all that is available, you can still vacuum bag, but you must still get the bag on before the resin gels. Therefore, apply the entire process to the first or second layer only. This will get time consuming and expensive, but it might be good practice (???). Use only non-coated peel ply in this case to avoid interlaminar silicone contamination. If there is no time to apply even one full laminate layer and bag it before the resin gels, I’d find slower resin, or skip the bagging procedure.
A good way to ensure full laminate cure before removing vacuum is to make a small laminate coupon on a workbench somewhere – a few square inches with an identical laminate schedule. When the coupon is cured, the boat is cured.
A partial solution for a slow vacuum source is to use a shop vac to draw down the bag. Before sealing the final pleat all the way insert a shop vac hose and pull as much air from the bag as possible. Then remove the shop vac and quickly finish the pleat seal. Apply vacuum source as normal.
I’ve assumed the use of a proper vac source – I know electric vac pumps can be expensive, but a good venturi vacuum generator can be had for less than $100. These connect to an air compressor, and can generate very good vacuum, albeit slowly. Do not attempt to use a shop vac or air compressor inlet as a vac source – these devices rely on airflow for cooling, and (if you do it correctly) a vacuum bag will ultimately have no airflow at all. There is a method for using an old refrigeration compressor however….
One last note – if using a vinylester (or some polyester) resins, you may experience styrene inhibition. This will cause you to remove the bagging materials and find what appears to be a sticky, under cured laminate. Fear not, with an hour or so of open-air circulation, it will cure tack free quite nicely. Assuming the resin was catalyzed and cured properly to being with, of course (see above note about a laminate coupon).
Bagging materials after the peel ply have a tendency to fall from vertical surfaces; keeping these dry materials in place can be a real pain. Vacuum bag manufacturers product various aerosol tackifiers that can be sprayed between layers to hold these materials together. It’s similar to spray on contact cement.
Above all, I recommend practice. Get some bagging materials, even a yard of each, and practice bagging something simple, like a phone book, to a tabletop. Better to learn slowly, when you are not concerned with resin curing.
Originally posted online at the Canadian Canoe Routes BBS in these three threads:
- Part I: www.myccr.com/SectionForums/viewtopic.php?t=16029
- Part II: www.myccr.com/SectionForums/viewtopic.php?t=16030
- Part III: www.myccr.com/SectionForums/viewtopic.php?t=16031
Posted here with permission from the author.