Cowling continued

I have finished extending the cowl surfaces to the table. This was a ton of mostly useless work. The plaster master was built slightly beyond the trim line, but the cheek cowls may or may not have been made long enough. I figured that I would just do a tangent extension of all the surfaces and then there wouldn't be any question if  the mold was big enough.

Extended surfaces

The surfaces were extended with Bondo. Like I said, a ton of useless work.




The next step was to seal the plaster. A couple of coats of Shellac thinned with Alcohol are brushed onto the surface. The dry plaster absorbs this like a sponge. Its important to seal the plaster so the release agents do not soak into the surface. My friend Duy came out Saturday to help. After the shellac dries a number of coats of Carnauba mold release wax is applied. Between each coat water is sprayed on and power buffed out. This is known as "spit shining" in the military. In reality its called water hardening. It creates a tougher surface finish for the wax and a release plane. I put 4 coats on the mold and hardened each of them. After the wax was applied, a fillet of Bondo was laid at the table base.

Duy adding Shellac

Bondo fillet

This is added so there will not be  any voids in the tight corner. The fiberglass mat will not lay up in a sharp 90 degree corner. The gel coat needs to have a solid backing or the surface will crack out. A void at the surface will cause gel coat cracking. After the fillet was laid, I scribed the master at the cowling trim line. Normally a mold is made such that both halves part at the same plane, but in this case, I want the upper and the lower mold to overlap each other by 1 1/2 " Both molds will have the same transferred scribe line for the final trim. I need to make an overlap flange for the lower cowl. In order to do this I want to lay up the actual part in the lower cowl mold. I will let that cure then I will put down a layer of silicon based peel ply and then lay up the overlap flange on the inside of the actual cowl. Since the cowl mold overlap the trim by 1 1/2" the overlay flange will be in the correct position. I will then trim the lower cowl and then bond on the overlap flange. This saves me 4 more intermediate tooling steps.
The next step is to construct the parting plane. I will cover this step with a little more detail because the clay trick is really neat and I want to show the detail of how to do it.

This is the beginning of the mold plane. I am using 1/8" tempered Masonite. I bought a sheet on Saturday and was planning to leave it in the back of my truck until I needed it on Sunday. On Sunday afternoon I went to retrieve it and it was gone!! I thought I was losing my mind too!!! Then I realized it probably vibrated out of the back of the camper shell on my truck. So I went to buy another one and sure enough, I found my original sheet busted up alongside the road out in the boonies. Because its not nice to "Mess with Texas" I cleaned it up and added it to my burn pile. Tempered Masonite has a nice smooth finish. The surface is still porous, so it too needs to be wax sealed. I rough cut the Masonite and you can see a few gaps between the Master and the template These are easily filled with modeling clay. the Template is attached to the master with small daps of Bondo. This is why the master was waxed with Carnauba wax.

Modeling clay

You can get this clay at any crafts store of fiberglass supply shop. being very generous, roll up some pieces and stuff it into the seal all along the edges of the template boards. Overfill all of the Gaps.




In order for the clay to stay in the gap, the back side of the board must have a tongue depressor hot glued to cover the gap.

Gaps covered

here you can see the tongue depressor covering a gap and the daps of Bondo holding on the board.












After the gaps are filled make a wooden scraper from a tongue depressor. sand the edge square and then add a sharp bevel.

Beveled tongue depressor

you can buy Tongue depressors by the carton from Aircraft spruce. Its cheaper that stealing them from the doctors office given the co-pays these days!!!
















After the gaps are filled its time to scrape away the excess clay. Take your time and slowly work into the clay lump. Use the template to keep the scraper flat and square to the surface, The results are very   clean and work quite well.

Before and After

This picture shows the clay before (upper) and after scraping (lower)

filled gap

this shows how well a large gap can be filled and why its not critical that the fit up be exact between the master and the template. The next step is to wax the template with Carnauba and then spray PVA release agent on the template and the plaster. I will cover that step and the mold lay up in the next update. I have to order the PVA so it may be a few days.