What is the Slickest Most Adhesion Resistant Prep for Sodium Silicate Core Boxes?

Here is a little more information concerning epoxy coatings and thinning them to make them easier to spray or to ease brushing them on.

Yesterday evening I used lacquer thinner to reduce West System 105/205 1:5 mix. I added 10% by weight of the thinner and then brushed it onto a small pattern with a chip brush. It brushed out easily without bubbles and hardened overnight to an extremely hard surface on the plywood and in the button left in the mixing container. It is not even remotely possible to dentist with a fingernail. Whether the resulting cured epoxy has the tensile and compression strength of unthinned epoxy I have no way to say. But I can say for purposes of coating wood patterns the thinning did not cause any evident reduction in surface hardness or smoothness. So, I would have no hesitancy to spray the mix thinned with lacquer thinner 1:10. That degree of cutting the mix did result in an obvious, but unmeasured, reduction in viscosity to a level similar to a lot of paints I have shot in the past.

Denis

 

Another good sodium silicate resistant coating seems to be polyurethane varnish. I recently used it to coat the inside of a sodium silicate corebox and it provided a smooth, hard, and cleanly-separating liner.

Tom Cobett mentioned how chemically reactive sodium silicate is. I had never thought about that. But experience proves him right. Shellac may be the worst coating I have used for SS though it works great for conventional green sand molds. It makes sense that shellac is reactive as it provides a base coat to which seemingly every kind of paint will wet out and adhere to. I use it to bridge over surfaces which may have been treated with silicone spray. Using shellac allows me to then spray lacquer, apply polyurethane, or epoxy with no or minimal silicone-caused fisheyes in a the surface. But SS sticks badly to shellac finishes. And lacquer also tends not to release SS reliably. You’d think good paste wax would make an excellent release interface over both shellac and lacquer, but SS seems to cut through it enough to cause poor release.

So, for now, my best setup seems to be either epoxy (probably best) or polyurethane varnish. (Catalyzed two-part polyurethane paint should also be very chemically resistant though I’ve not tried it.) and To those coatings I add paste wax liberally applied but then buffed out and then a dusting of graphite or talc. If you don’t buff the wax at least lightly, it does not release as cleanly, BTW. Those combos result in crisp clean release of the SS cores after only light rapping.

As an aside, I will soon start a thread on making steel anchors to be used for bolting into SS cores.

FWIW.

Denis

 

You probably have good reason to believe otherwise, but I do not out and out try to cause trouble with materials incompatabilities So, no, I have not tried it with K-Bond/P-bond. I would fully expect the oil-bound sands to soften it and possibly melt inot it. I do not usually use it as a finish coat though I know that classical pattern makers did and do. I have used it as a finish coat with green sand and, as you would expect, it worked fine. But I prefer lacquer for a finish coat in most circumstances. Not for SS, though. West 105 thinned 5 to 10% with lacquer thinner can be applied nicely with no evident hardness loss when brushed as a finish coat that is nearly inert. Sprayed Rustoleum epoxy sprays on very nicely but does not get super hard. Gloss polyester varnish is pretty inert, brushes on easily, and gets very hard as well. The epoxies seem to take 12 to 24 hours to cure pretty well and the polyester takes maybe 8 to 10 hours at low 70'sF.

Denis

 

Here is a little more follow up on using polyurethane varnish for SS patterns. I have made a half dozen 6-pound SS molds using a box that was polyurethane-coated as it’s final coating. I wax it with Johnson’s wax and dust it with talc or graphite each time. I have not tried using it without applying wax. It has worked perfectly. The polyurethane does not allow the SS to bond at all. Epoxy is similarly resistant but not as easy to apply and does not seem to be in any way superior. Aerosol epoxy makes a softer finish than the polyurethane and is therefore inferior in my view.

My conclusion, for now, is that polyurethane is a simple and effective final finish for SS core boxes and will be my go to.

Both lacquer, and even more so, shellac will allow bonding of SS cores to the box even when waxed. So, I would strongly suggest not using them as a final coating for core boxes to be used for SS.

Incidentally, flushing polyurethane varnish cans with Dust-Off or similar relatively inert gases prevents skinning of the varnish (and any other type paint or varnish) remaining in the can between uses.

FWIW,
Denis