I've used the patching stuff on a ball and other things.
About 1/2 hour after trowelling it on, I dip a rubber gloved hand
in water and rub it smooth.
You get an even, grained finish.
I suppose you could polish that again with a piece of hard plastic,
if you want it less grained, though I've never tried.
Someone else may know?

HI,
I have found the patching stuff to be extremely sticky, and for me I like a smooth finish, although the grain is fine. I ahve also found that the patching stuff sets somewhat quickly so you may have to have a file to file down high points before Linnea's suggestion. Once you have gone over it like Linnea has recommended with a wetted hand you can then let that begin to set while you mix up a slurry of portland cement and acrylic fortifier or bonding agent, and then either hand trowel it on or using a rubber spatula/or kidney rib you can apply it over the grained surface. This will fill in all the little voids, will add weight and if your previous surface is not dry or set enough it may cause this last layer to suface crack. At this point you will want to keep the area away from wind and high heat, which will create a better atmosphere for crackless surface.
Another option is to consider what sort of color you may be adding and it may take away the grain to the smoothness you are desiring.
Happy creating, spring and new ideas abound,
Dena

You can get a finish like polished marble if you take the time and do the work. There are several approaches but they all involve basically polishing the surface. This can be accomplished regardless of the aggregate, but the finer and fewer...the easier the polishing is. Large, hard aggergates require very aggressive polishing. The beautiful Terazzo floors that have come back into fashion lately are an example of big, chunky aggregate in a mortar bond that is highly polished. But a combination like that requires a diamond polisher and a lot of work. Reducing the aggregate to extremely fine sand or a "microaggregate" such as flyash permits polishing with sandpaper and a great deal less elbow grease. Cement pastes are relatively easy to polish...it's the aggs that are as hard as...well, rock. 'Cause thats what most of them are. Little rocks.

You can build up layers of a paste-type mix on a 3-D shape by actually painting them on until you get the thickness required. However...curing must be under absolutely ideal conditions or it will crack to pieces. Pastes must maintain near perfect hydration throughout the entire cure or they will fail. But if you can overcome that hurdle, you can start with an old wood rasp to knock off any high spots and then work your way through progressively finer & finer sand papers until it has a mirror finish. Wet sanding is generally easier and is much less hazardous because it eliminates the very fine and dangerous dust.

Best of luck and be sure to let us see the results no matter how you wind up finishing it.

Morning!
Spheres!Alright,another person who thinks orbically(new word,eh?).
Laura and I(but,Laura started me down the trail,her fault!)have exchanged ideas about the making of orbs.
She got on to the idea with Quickwall and detail mesh(EIFS in the U.S.).Slapped a coat on,put on the mesh,mashed it down to set the mesh and then another coat(Laura,sorry if I'm wrong;memory goes with age!)to finish it.All very light and,I would think,easy to smooth either by sanding or glove finish.
Not having Quickwall available,I created my own formula,which,so far,after 6-8 months,works just fine,thank you very much!Equal parts Portland,fine white slica sand(about 130 mesh);either silica fume or metakaolin,at a dose of 8% to the weight of the concrete,fibres,and acrylic admix,no water.This mix sets very quickly,so don't go slow!The beauty of it is that you can do the whole ball(I've done a 26 inch ball)without having to wait.This stuff("Mymud")will adhere upside down!I've smoothed a smaller ball by sanding,the bigger ones(I want to go to about 40 inches)I'll trowel finish,or some other "artful" way.
I've had a smaller ball with water in it in our home(put a mister/light combo in it-hypnotic!)for 6 months.Solid as a rock.
I use this mix to make lighter masks,septic tank covers.
Give either formulation a try.You'll be happy with the results!
Cheers from the sunny West Coast of BC.
Rick

Hi gardengal
If I want a smooth finish I use a nice big sponge with very fine holes. These are sold in concrete stores for a couple dollars or a good bath sponge might do
After applying your crete/tufa take a slightly damp big sponge and dab firmly the entire surface. Dab only do not rub. The sponge will level out bumps and ridges and leave a smooth surface behind. Keep dabbing until the surface looks even. It will be a very even surface when you are done. If you want baby bottom smooth wait a couple days and sand lightly with a sanding sponge. You can buy them anywhere. Buy a couple a 50 grit, and 80 grit. Have a bowl of water handy, wet your sanding sponge starting with the 50 grit and scrub sand your ball. If this isn't baby bottom smooth use the 80 grit.

If you A-Sponge your ball smooth then B- wet sand it you will have a smooth even surface.
The entire sponge sanding process adds just a few minutes work to a 16 ball and you will be amazed at the result.

Good luck!

Thanks for the great replies. You folks are the experts. How would an individual obtain flyash? Sounds interesting but probably comes by the truckload. Thanks again.

I have made balls using a 24" kids playball. I made my concrete mix thick like clay. It clung to the ball nicely. I let that dry overnight. I applied the mesh dipped in a slurry, then let it dry. For the third layer I have found that making a thickish slurry and painting it on the dried layer works wonderfully. After setting slightly, I wet a sponge paintbrush and smoothed it out. It looks great. It has a solid but not heavy feel. I am almost ready to add a decorative layer then paint. I looked for Quickwall, but it's not available here. There is a concrete patcher that comes in small 5# bags. I wonder if it is the same.