Monday, March 2, 2009

An opera in 3 pots.

Never made a gas tank before. It's obvious I'm way over my head here. Anyway, let's begin making the top part.

I chose to use four facets of glass per layer. As luck would have it the tank is reasonably symmetrical, so the pattern consisted of 2 facets, roughly looking like this:

Keep that mildly pissed off woman's expression in mind, because by 8:00pm yesterday, I was right with her, maybe even several lengths past that, ready for a bucket of plonk and a flintlock to point at strangers.

Making the patterns is pretty easy, just hanging some news print off of the plug, and cutting notches around the compound contours. Kind of like so:

(I like how her picture ended up upside down...perhaps that's why she's giving me that scorning look of disapproval -- as if my shenanigans have no purpose to her discussion on philanthropy for the Arts in the post-Bush, pre-economic apocalypse).

Layup for the top was a complex affair. I knew I'd take too long to get it all in one pot of resin before it started to gel. Even with the slow hardener, after 30 minutes resin begins to cure really fast when it's amassed together in the mixing dish. I carried forth the layup in these overtures:

  • First a topcoat of black tinted resin right on the mold that I'd let cure to the point of tackiness. This from an idea taken directly out of Tygaboy's advice on Bay Area Riders Forum to achieve a smooth outer surface and facilitate adhesion with the first layer of glass.
  • Then mix some milled glass fibers with more black resin to fill in the cracks/crannies around the fuel cap recess, as well as the dimples around the shoulder of the tank that are there to: a) clear the fork triple clamp bolts, and b) make it harder for the glass to lay down smoothly.
  • While the topcoat dried to a tacky consistency, I cut the glass for the main facets. Once cut, I mixed the first pot of tinted resin and went at it. After a couple layers the resin started to get kind of snotty, so I found a stopping point, ran inside and took the turkey thighs I had roasting out of the oven, and mixed a pot of clear resin to finish out the layup.
  • Vacuum bagging. This exercise was fail from the get-go. First, I didn't cut the peel-ply big enough to extend past the overhang in the tank -- crucial because I wanted to tape it down to the outside of the mold. I then had a few wads of breather cloth jammed into the interior, some bunched up into the shoulder and under the overhang to help apply pressure from the bag. I then stuffed all this into the bag and began pulling air with my bicycle pump. After not long, it was apparent that there was not enough surface area of the bag to reach into the overhangs of the tank *and* all the way into the depths of the shoulder of the tank, even with all the extra wadding I had used to take up these areas. I decided to quickly pull the layup from the bag, remove the wadding, peel-ply and just let it cure on its own.

Six pairs of rubber gloves later, and this is what the place looked like last night. Note the brush right in front of the mold, I sawed off the handle for easier maneuverability within the mold:

Yeah, vacuum bagging is a nice to have thing, and a lot of composites geeks use it to consolidate a layup and get the optimal ratio of resin to reinforcement for the lightest possible construction. I've never been as interested in that factor as much as the way vacuum bagging helps the fiberglass to wrap around complex edges. Since I'm laying glass on the interior of the mold, it actually seems to have laid down pretty well. I'll leave it and see how it comes out of the mold in a day or two.


  1. Hi, I like your blog, I am actually starting a similar street tracker project so it is really useful for me. I have a 1985 XL600R and I am in the process of modifying the frame. Strangely enough some of your pictures look just like mine, I started sculpting the tank with foam just like you did but I ended up finding a tank from Omar's web site which fit perfectly. I will start a blog this weekend and start posting progress photos. It would be great to share some of our learnings.

  2. Hi Thierry! Definitely share your blog site once you get it set up and I'll make sure to link out to it from mine.

    I will be very curious to see how your work progresses!

    Good find on Omar's fuel tanks. Which one works for our bike?