Making a vent hole in fiberglass

[Jim Garry]

The vent hole shown in the photo is something I've seen in several cars of different makes. So it appears not to be manufacturer related but might be a standard type of reinforcement technique when needing to create a big vent hole in fiberglass bodywork (maybe). Can anyone shed any light on how to create this? Is it hand made or is there something that can be used as a template of sorts?

Thanks for any advice!

[billtebbutt]

We just did a similar job, except the hole was larger and we were making a fibreglass "bubble" over it. My friend Steve B is a genius at this stuff, and made it look easy.

1. Cut hole in engine cover
2. We taped blue stiff foam insulation board pieces on top of the headers as a base to build off of. Since you are just venting the under deck arear and don't need to "clear" say a set of protruding headers, you could build off the underside of the deck cover
3. Shaped the foam with a file to get the bubble shape we needed
4. Used muffler tape (shiny surface), to cover the shaped foam and this tape extended well over the paint on the bodywork. This acted as a "release coat, if you will. When this was done, we had the look of the basic bubble completed
5. Used really thick/stiff cardboard, and build a "dam" around the outside edges of the bubble. Now we had a vessel to pour expanding foam into
6. Poured the foam into the vessel. DON'T POUR TOO MUCH, or you'll have a hell of a mess. When cured, we carefully forced the female foam shape off of the male "bubble" shape. Now we had a female mould
7. Once we smoothed out/sanded/filled the mould, we laid in a proper chemical release coat.
8. When that cured, laid in a coat of gelcoat
9. When that cured, we laid in a couple of layers of glass

Voila, we (truthfully, Steve B) pulled out a beautiful piece of bodywork from the mould. We chose to make it removable form the engine cover, so it is affixed with a couple fasteners. We could easily have trimmed it to fit, and glassed it in permanently though. It was a lot of work, and Steve did most of it, but without him i wouldn't have been able to think my way through to do it. Having done it once, I am confident i could do it again on my own.

One thing we learned was interesting. We poured the foam in two steps as we needed more to fill the vessel. This was a mistake, as it seems as though, even both pours were liquid when they were together in the vessel and were mixed only a couple minutes apart, they expanded at slightly different rates and seemed to "push" each other away from each other where they met in the vessel. This resulted in some unexpected gaps in coverage, which meant Steve had more work to do on the male plug later on to get the finish nice.

Good luck. It is a fun project, and I learned a lot!

cheers,
BT

[S Lathrop]

That detail on Jim's car was done on the original body. The buck for that body was done in "studio clay". This is just modeling clay that is quite stiff at room temperature but when heated to about 120 degrees or a bit less can be worked by hand and clay modeling tools to produce the detail you want to add to the body.

I have used this many times to create some detail that I want add to a fiberglass body. You can blend the clay to the body and smooth the clay surface to where I used that as the buck and make the mold over the clay and some of the existing body. cover the body and the clay addition with a parting agent so you can easily separate the mold from the existing body and the clay. I have heard this called a skin mold.

Next step is to trim the body back to where you can lay up the fiberglass for the modified body and have a smooth transition from the original body to the new detail

I have found that studio clay is the easiest and for me the fastest way to make changes to existing bodies.