BTW, those surface coat failures look to be caused by fabric bridging the corner. The best surface coats in the world will fail like that if there are voids between the surface coat and the reinforcement.
Also, to clarify, you must spray the PVA on top of a wax. You don’t use the PVA alone. Wax the mold then shoot the PVA. You many know this but I couldn’t tell from the comments made. The PVA will give you an easier release than just wax. Once a mold is conditioned you can wax once every 4 or 5 cycles with PVA being used each cycle.
You could also move to a semi-permanent and do away with the wax and the PVA. This will speed up your production and could improve your surface finish. Semi’s go on very easily and quickly. They will also replicate your mold finish unlike the PVA.
If you part size isn’t very deep or very large you could just cast the back right over your surface coat and forgo any reinforcment. Mix oven dried playground sand and Adtech EL-302-PC to a slurry consistency, pour it over the surface coat to about 3/4" and be done.
Here is a qoute I recently made in another thread:
I make my own surface coat using my chosen laminating/tooling epoxy resin and a fast hardener. I mix in 27% West System 404 High Density filler and 12% graphite powder by weight. I brush this first coat on rather thinly (dimensional), wait 10 minutes, and then zap it with a torch. The torch super heats the air bubbles and causes them to pop. I let this first coat tack up (tacky to touch but doesn’t transfer to finger). This I use the same resin and wet-out lengths of carbon tow. I work the tow into any tight corners with the tip of a small acid brush where fabric bridging can be an issue. I usually use 3 pieces of 12K in the corners. Then immediately I’ll brush on another application of the surface coat, wait 10 minutes, and zap it with a torch. I’ll then sprinkle this with cotton flock and blow off the excess. If the cotton flock seems to disappear I’ll sprinkle it again. I do this by putting some flock on a piece of screen. Then I just tap the screen to dispense the flock. At this point you can either stop and let it cure or continue with the reinforcement once the second application of surface coat has gone tacky. The flock will make for good secondary bonding if you decide to take a break. Depending on the part depth I’ll continue the process in the following ways.
If the part depth is rather thin then I’ll brush the surface coat with resin and then I’ll just mix Adtech EL-302-PC low shrinkage laminating epoxy and oven dried playground sand into a pourable slurry. I’ll pour this slurry over right over the plug so everything is nicely covered leaving a flat surface. The sand will sink and the excess resin will rise to the surface. There will also be a lot of bubbles that will rise to the surface. These can be simply popped with the torch. If you mold is small you can be done at this stage. If the mold is larger or requires more strength then I’ll use the excess resin to wet out around 30 ounces of glass. This goes really fast since the surface is now flat. Then I’ll mix up some sand and epoxy to a very stiff consistency and pack about 3/4" on top of the glass, then add 30 more ounces of glass. The glass weight and orientation should mirror the first layer of glass. The advantage is that this system poses no risk of fabric print-thru to the mold surface and the laminating is super easy since it’s performed on a flat surface. The downside is that these molds can be rather heavy and it requires a forming damn around the perimeter.
