This is not the way a vacuum bagging project is supposed to look. The sandbags are the giveaway; they are a resource to turn to when trying to stave off disaster.
I began this phase of the project excited to finally have begun the process of rebuilding the boat. My shop mates quickly noticed I was up to something unusual, so I also had a curious audience.
Making a vacuum bag is the best method for installing planks on the inside of the boat. Done right, the bag provides a clamping force that distributes pressure evenly. I thought I was being conservative by confining this step to six planks. A professional would try to do the whole hull, or at least half of it, in one shot. As it turned out, though, I should have been even more cautious.
I outlined where each plank would go and I numbered and marked each one. Then I mixed a batch of epoxy and used a two-inch paint roller to spread it first on the hull and then on the underside of the planks. I mixed a second batch of epoxy thickened with a filler and spread that on the hull with a finely notched plastic trowel. (The unthickened first coat seeps into the wood; the thickened mixture provides a paste between the boards and the hull, helping to fill out any hollows and to avoid voids in the laminate.)
Mixing epoxy is a bit like firing a starting pistol. The moment you blend the resin and the hardener, a chemical reaction begins that eventually turns the liquid into a hard-as-steel solid. Heat accelerates the chemical reaction. My shop thermometer showed it was 87 degrees, so I used West System's slow hardener to give myself more time. I figured I had at least an hour, maybe 90 minutes, if I got the mixture out of the pot quickly and spread out on the wood (the curing reaction slows when the epoxy molecules are distributed at greater distances from each other).
I like to think of myself as cool under fire, but for some reason, epoxy always stresses me out more than it should. Feeling under the gun, I sometimes work too frantically. And that's how I made my first mistake.
Midway through applying epoxy to the planks, I misread the markings I had written on the boards and coated the wrong side of three planks. That meant that I had to go back, turn them over, and coat the other side, too. Now I had three super slippery planks that would be difficult to wrestle in place and staple down.
Once that was done (after going through three pairs of disposable gloves), I placed a layer of release fabric (a material that doesn't stick to epoxy) over the planks. Next came a layer of a breather cloth that would enable air to flow within the bag. Last was a thick plastic sheet that would serve as the vacuum bag. I had cut all of these materials to size beforehand. I also had already surrounded three sides of the planking area with a clay-like adhesive. The adhesive is what holds the plastic bag on top of the breather cloth, release fabric, and planks. It creates a seal when air is removed from the bag. In the best of worlds, the resulting vacuum gives you the equivalent of one atmosphere of pressure (more than 2,000 pounds per square foot) spread evenly over the bag,
I initially put down only three sides of the adhesive because the fourth side had to run on top of the planks themselves after they were in place. As soon as I started putting the fourth strip down, I recognized the makings of disaster. The clay refused to stick properly to the three planks with epoxy on the wrong side. I quickly wiped as much off as I could and did my best to try to get the adhesive to adhere.
The last step in vacuum bagging is to run a perforated hose under the plastic bag, attach it to a pump, and start removing air from inside the bag. The bag needs to be leak-free to remove all of the air. When I flipped the switch on the pump, I could hear hissing. A lot of hisssssssssssssssssing, in fact. I had a typhoon of leaks. Most were in the area where the fresh epoxy was keeping the adhesive clay from sticking properly. It's not unusual to have a few leaks at first, but this was way worse than usual.
I spent the next 20 or so minutes frantically trying to plug the holes, using more clay, tape, and plastic. But it was a losing battle. When I only had about 10 minutes left before the epoxy started to "go off" or harden, I got desperate and started running around the shop looking for a solution. At the last minute, I found four 50-pound sandbags and set them down in the area where I was getting the most leaks. At the very least, I surmised, the sandbags would apply uniform pressure on the wood the same way the vacuum bag was supposed to, though at about a tenth of the force.
But .... I also got lucky: the sandbags greatly reduced the amount of leaking. A decent, but not great, vacuum developed inside the bag. As an added measure, I fastened wood clamps on the top of each plank.
Then I collapsed on my shop stool, exhausted, drenched in sweat, and disappointed. And that's when the ribbing started.
"What do you got going on there, Mr. Boatbuilder?", one of my fellow woodworkers asked as he passed by.
"Preparing for a flood?" remarked another.
"Oh, so that's what a vacuum bag isn't supposed to look like", said a third.
Ha. Ha. Joke's on me.
In coming posts I hope to be able to show that I regained my composure, learned from my mistakes, and executed a much better vacuum-bagging operation.
Below is a photo I took the next morning after I removed the bags and adhesive, sanded the epoxy off the top of the three planks, and did a little filling between the seams. They still need a fairly aggressive hand sanding. But, in the end, the sandbags did their job. It wasn't a complete disaster.
Note: the red Bessey clamps are holding the hull in the mold. The new planks should help the boat regain some of its designed shape.
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