After sanding and filling until I was blue in the face, I primed the plug using an Evercoat brand product called Uro-Fil. It's a high-solids (hi-build) sandable primer. It always does a good job of showing me that I did a bad job of filling all the pinholes!

The whole point of all this sanding, filling, priming and smoothing is that the finished fiberglass part will only be as smooth and nice as the plug it's made from.

This step about killed me. After a couple of layers of primer and sanding it was time to cut the plug into 2 pieces. With this method of fabrication, there's no way to remove the finished piece from the plug unless it's in 2 pieces.

Tony Bengalis describes a "disappearing foam" method where you lay glass up on top of a foam plug and then dissolve the foam using solvent once the resin is cured. This method wouldn't work in my case because we're going to use pre-impregnated cloth, vacuum and a baking oven. The combination of the oven and the vacuum dictated a solid plug--so the disappearing foam method wasn't appropriate...

I glued a 1/2" thick piece of MDF to the "lid" to finish off the plug and so that the finished fiberglass piece will be slightly bigger than we actually need it. We'll basically trim off the 1/2" extension for the finished part.

I used a router with a flush cutoff bit to trim the MDF exactly to the contour of the lid. I had never actually used a router before this. I found out very quickly that routers are outdoor toys if at all possible. I couldn't believe the cloud of sawdust that router spewed out! After about the first 8 inches of cutting I moved the whole process outside where 30 knot winds took care of the sawdust!

The main body of the tank mold required a flange approximately .050" deep and 1" wide. To achieve this, I used a slightly oversized bearing on the router bit and let it fly. Because the cutter portion of the router bit was slightly less than 1" wide, I had to first glue 3/4" MDF, cut it and then screw on an additional sheet of 1/2" thick MDF and cut it using the flush-cutting bearing on the cutter bit.

You can see where I've started applying Bondo to feather the transition into the flange.

After significantly more Bondo and more SuperFil and more sanding, I was ready to prime again. Here are the parts after several coats of primer sitting just outside my hangar door.

Unfortunately, even after several iterations of sanding, filling and priming, I still found a few pinholes. I'll do one more focused round of Superfil filling and sand one more time before what should be the final coat of primer.

If all goes well, I'll have the plug finished by Monday evening. I'm not saying which Monday... just Monday. :-)

The "lid". This is foam, 4 layers of fiberglass, about 2 pounds of Bondo, a little Superfil, 4-5 coats of sandable primer and then about 3 coats of white/grey finish coat. After this, I color-sanded (wet-sanded) with 800-grit, 1500-grit and then rubbing compound. I then applied several coats of industrial-strength sealer and then 4-5 coats of mold release. Not exactly a speedy process...