Well...…. Technique is a bit generous of a term but in this case I was basically making a box with two open ends so I formed a box out of cardboard covered it with duct tape (resin does not stick to duct tape) then covered the box with a couple of courses fiberglass woven cloth and let it harden. After that I pulled all the cardboard and tape out of the box, cleaned it all up and continued to add squares of core matt to give it strength then covered it with 3 or 4 more woven cloth courses. I did not use chopped strand for this because I just didn't but chopped strand could have been used to add thickness instead of more courses of woven cloth. I had bought a big roll of leftover cloth really cheap many years ago so I tend to use it because I already have it.
Now I made the flat center vane on a flat piece of wood covered again with duct tape in the same manner as the box. 2 courses of cloth coved with a core piece and two more courses of matt. Once hard used a template made from, you guessed it cardboard, to shape the rough flat vane. I then held it in place with...………... ya duct tape, and used matt to permanently set in place and let the one side harden. Now I removed the duct tape on the other side and glassed it in place with more woven cloth matt like the other side.
Fillets an be formed at the right angle point to get a nice transition by mixing resin, hardener, milled fiberglass, and microspheres. Mix it up in small quantiles to the consistence of peanut butter. This can be used as an adhesive also.
The real fiberglass guys are cringing about now...………….but you can get good results this way if you need to make one off pieces that are not structural in any real sense.
Get two at least different woven cloth weights. 2 or 3 ounce light weight to form corners and 6 ounce to make the flat bigger pieces. I use polyester resin, it is the least expensive, and the hardener that goes with it. I also buy very cheap acid and chip brushes in large quantiles and tend to throw then away at the end of the working session.
Mix small amounts, like a couple of ounces or less at a time and finish a course or two at a time, then get set and do another course or two with the nest batch. Cut all the cloth in the sizes and pieces you will need before you mix resin. Resin costs more than cloth. And buy cloth that is at the bottom of the scale when it comes to cost per yard.
I am speaking from a one off small projects perspective not a large piece thing like a body/mold. I would not know how to properly do something like that without some real instruction.
Here's a supplier that has all the stuff I mentioned above, Harbor Freight for brushes, and another source for instruction on all things fiberglass.
I relied on Howard's earlier post pretty heavily to help me fabricate my duct. Howard's technique is actually a little cleaner than mine . I specialize in low skill fiberglassing!
I used poster board and packing tape to make my molds and laid up on them as Howard did. Separating everything is pretty easy because you can just pull and cut everything away without worrying about draft angles - so you can make as complex a shape as you want.
Posts 28 & 35 on my blog summarize how I made the basic shape and final exit/turn. I hadn't figured out how to make that transition when I made the initial piece so I relied on my final bodywork to help form the last turn out of the body. I added insulation to the interior of my box as well as a healthy amount to the interior of my foot box. I have zero heat intrusion issues. As Howard says, I think having such a duct is a huge step toward managing foot box heat.
I went back and looked at my construction process for the radiator duct. I am now pretty sure I used a release agent painted on a poster board that had a political ad on it thus making it a bit non porous. Later on I made a couple of things with just duct tape and found that I could skip the release agents on stuff that I didn't really care how it looked once complete. Interior ducts, etc.