This project has seemed to taken on a motto - "it's all a learning process". This couldn't be more true for the mold.
I have been working with fiberglass and carbon fiber for years, but everything I have done in the past has been with molds that were either CNC machined, or pre-made with some other means. This is the first time I have ever made my own mold from scratch. It took 4 tries to get a usable mold, but I learned a lot in the process that will help me with future projects.
Failed attempt #1
For the first attempt, after the plug was sanded and smooth to my liking I hot-glued it to a piece of plexiglass. This was waxed and sprayed with PVA. Then I sprayed normal Tap Plastics Gel Coat. I used a regular HVLP paint spray gun, and thinned the gel coat with acetone for spraying. The gel coat doesn't spray well with this gun, but I gave it a try anyways.
After the sprayed thin layer of gel coat had set up a little bit, I brushed on a heavier layer. I let this setup completely and then fiberglassed using chopped mat glass and epoxy resin.
This is where I screwed up. Without sanding the gel coat (a polyester resin) to help with a mechanical bond, I was relying on a chemical bond. But unknown to me at the time, polyester and epoxy resins don't form strong chemical bonds.
The next day I tried to pull the mold off the plug, which popped the hot-glue right off the plexiglass. I struggled to get the plug out of the mold with plastic putty knives, got frustrated and went at it with a flathead screw driver. This started chipping the gel coat, so I had ruined the mold at that point. Finally I worked the plug loose, and found that where the gel coat had been chipped, I could pick at it and it would peel off (because of the lack of chemical bond between the incompatible resins).
Failed Attempt #2
The second plug was made much the same as the first one. Again it was hot-glued to the plexiglass. I sprayed the gel coat the same way, but I thinned it a little more this time with the hopes that it would spray better. It sprayed just as poorly, but I let it setup anyways. I did not brush a second layer on this time, so the gel coat was thin. I then fiberglassed it, using polyester laminating resin from Fiberlay. Only a few minutes after laying the fiberglass, I noticed much of the gel coat had lifted from the plug/plexiglass, much like paint orange-peeling.
The faults here were - too much acetone in the gel coat, too thin of gel coat, gel coat not allowed to setup long enough, possibly too much catalyst in the gel coat.
Failed Attempt #3
This third attempt didn't even make it as far as laying the gel coat. I decided to can using hot-glue because I wanted the plug to stay attached to the plexiglass. This was for more insurance, that if the mold did not turn out, I would still have a usable plug for another attempt. With it popping off with the hot glue, the plug would get destroyed required a new one to be made.
Another thing different with this 3rd plug, was when fiberglassing the foam plug, I vacuum bagged it. This gave me a much better starting point so the sanding/bondo work was MUCH faster.
Now comes the idiot part. I decided to stick the plug to the plexiglass with epoxy for a good permanent hold. First I sanded the plexiglass thoroughly for better adhesion. Then - the dumb part - I decided I wanted more surface area in the foam to hold it better. So I used the angle grinder to gouge the bottom of of the foam plug, filled this cavity with epoxy, turned it upside down on the plexiglass. All looked great. Until I noticed all of the epoxy started running out from underneath. There was no way to save it, so I just let it run out and setup. It would probably be usable, but there was epoxy all over the plug, and would have taken a lot of touchup sanding and rework.
Successful Attempt
For the fourth (and last) plug I decided to make a drastic change to the design of the mandolin. Instead of the very round gourd-like shape of the body, I made it much more shallow, with more of a flat arched-back. This was visually much more appealing, and seems as though it will be more comfortable to hold when playing.
The plug was made the same way as the third, by vacuum bagging the fiberglass, so there was very little sanding/bondo work.
This one I sanded the plexiglass, but did not gouge the bottom of the foam. I also thickened the epoxy with viscofill thixotropic powder from Tap Plastics. This would help prevent the resin from running out from underneath the plug. After this had setup, I was not happy with the sharpness of the corner between the plug and plexiglass, so I put a VERY small bead of clay in the corner, just to round it out slightly.
The plug was waxed thoroughly using Fiberlay PartAll #2 mold release paste, and then sprayed with PVA. This time I also used Fiberlay Tooling resin, which is a Vinyl-Ester gel coat designed for mold surfaces (hard, durable, scratch resistant, and shrinkage-resistant). I decided not to thin and spray the tooling resin, but instead to brush it. This put me at risk for air bubbles, but luckily I brushed it properly and avoided that disaster. I didn't get the tooling resin very even but I figured this would be ok (and it was). I then did two good layers of fiberglass when the tooling resin had just started gelling, to ensure a good chemical bond.
After everything had setup I had to pull the mold. Unfortunately even with the sanding I did of the plexiglass, it still popped right off. Since I knew I'd lose the plug, I did it as safely as possible to save the mold. I did this by using a knife to gouge out all of the foam in the plug, leaving mostly just the fiberglass shell. I had to do some ripping and tearing, but the end result.... was a perfectly smooth mold!
