Lets talk about
filler hardness.
Durham's Water Putty:
Durham's putty is as advertised, which is "Hard as a Rock".
Durhams is so hard that it is very difficult to sand it down without damaging what is around and under it.
DAP spackling compound is very good filler material, but it dries out in the container before I get a chance to use it more than one or two times.
The material I use now is Fastpatch 30, also a DAP product, and comes in powder form, and is mixed on demand.
On an unscientific hardness scale of 1-10, Durham's is a 10 in hardness, and not really of much use as far as I am concerned with filling patterns.
Durhams is so hard and inflexible that it can expand and contract at a different rate than what it is applied to, and can chip/flake off.
Durhams is like concrete; once it sets, you are pretty much done with it.
DAP Spackling Compound:
DAP spackling compound is about a 2 on the hardness scale, and so very easy to sand, and easy to level with a soft surface without gouging into the surface. I coat the sanded spackling compound surface with shellac to give it some surface hardness.
The downside is that the shelf life in the container is way too short.
It must be used in perhaps a week after purchasing it.
The Fastpach 30 wall filler is very good material.
It is easy to mix, sets quickly, and is nice to work with.
The downside is it is a little on the hard side, and more like a hardness of 6.
It is not excessively hard, but is about twice as hard as it needs to be.
I filled the inside of my hopper pattern halves with a rough coat of Fastpatch, just to make it easy to pull these patterns from the sand.
In some places the filler is 1/4" thick, and this requires a surprising amount of work to sand out flat, even when using a coarse 2" sanding disk in the variable speed drill.
Fastpatch is very tough material, and it feels a bit more flexible than spackling compound.
Fastpatch will take a lot more abuse than spackling compond after it hardens, but I will still coat it with shellac.
Sanding Fastpatch takes 2-3 times longer than sanding spackling compound.
All of these products are waterbased, and I use these products because I can use them inside without annoying fumes.
Bondo:
I have used Bondo, and the fumes are bad.
Bondo is too hard for my preference, and I consider it about a 7 on the hardness scale.
Bondo is somewhat flexible, which can make it a bit odd to sand if it flexes under the sanding tool.
Bondo costs a lot more than wallpatch material, and it tends to get on everything, permanently.
The cleanup with waterbased fillers is very easy.
Skim Coat:
Automotive skim coat is like thin bondo, and I have tried two types of this material.
I was not pleased with the skim coat.
It sands like bondo, which is it has a bit of a rubbery feel to it, and no doubt make to be a bit flexible for auto body use.
On a hardness scale, I would say skimcoat is about like bondo, and a 6-7 on the hardness scale.
Wall Patch:
So far the Fastpatch 30 is working pretty well manually applied as a thick putty on the inside of the pattern halves.
I will use it on the outside of the coreboxes also, to basically insure that these items can be pulled from the sand when molding for permanent aluminum patterns.
The interior of the pattern halves, and the exterior of the coreboxes does not have to be perfect, and so a coarse 2" disk in the variable speed drill works pretty well in leveling out these items.
For the exterior of the pattern halves, and the interior of the coreboxes, I am going to try spraying on Fastpatch 30 in a thinned form.
I only need a very thin coating, but the key is that the coating has to be very even, so that when I sand, I don't gouge through the filler and expose the plastic lines below.
PLA Filament (3D printer filament):
PLA plastic is quite hard, and I would rate it perhaps 9 on the hardness scale.
PLA cannot easily be power sanded, since it tends to melt and streak, instead of getting cut cleanly.
PLA is not easily hand sanded, but it can be done.
PLA can be buffed somewhat smooth with a 2" fine sanding sponge operating at a very low rpm, with is good for an initial smoothing of the surface and edges.
I prefer patterns made from wood, but the advantages of 3D printed patterns make PLA the pattern material of choice for me these days.
PLA has little or no odor while 3D printing, and that is a real bonus for me, since some of the filament material I tried years ago had very noxious fumes.
Once the filling work is complete, I can start thinking about casting permanent aluminum pattern halves and corebox halves.
The gate/runner prints are ready for use.
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