I do not cast leaves so there could be chemical interactions I'm not aware of but I suggest you try to find a bag of CRYOGENIC Perlite.
This stuff is like sugar and as to weight you can fill a huge rubbermaid container with the stuff and you cannot tell the difference between the full container and the empty one by picking them up.:)
One Portland
One 00 sand
Two Cryogenic Perlite
Acrylic admix or Latex paint to 50% of your water.
This might give you exactly what you want. A durable light weight finish.
Cryogenic Perlite is worth looking for because by volume it is actually cheaper than sand. 3 cubic feet for around $15.

In my experience, you lose all the fine detail with hypertufa. I have put a 1/2 inch layer of concrete on the leaves and left the back roughened, came back next day and put metal stock fencing over the back of the leaf and put a 2 inch layer of hypertufa over that. It gave me a lighter leaf, but I saved the detail. I haven't painted any leaves, so can't help you with that. BB

Isn't losing the fine detail the whole point of Hypertufa?
The peat rots out leaving a rough surface and also a weakened product.
Can someone explain why you would put tufa on the back of a leaf knowing over time it will deteriorate as the peat rots?
Wouldn't light weight concrete made with Perlite or vermiculite make more sense?

Janice, how much do your leaves weigh now when you make them out of your portland/sand/ mix ? I would estimate that my elephant ear leaf casts (about 22 inches by 14 inches) weigh under 5 pounds. If that is lighter than what you are used to you might want to try a bag of Quickwall by Quickcrete. It has fibers and a bonding agent in it and makes very thin leaf casts.
I also like using it to smear a quick coat on leaves that are not very "cooperative" to cast (cut-leaf philodendron)... then I pat a thick coat of tufa all over the leaf and let if get in between the separate leaf parts and I make it surround the whole leaf in some shape like a big circle or oval. When it dries you turn it over and you have a white cement print in a tufa circle. These usually turn out great and it saves a lot of weight compared to solid concrete.
Laura

Hi Laura
Quikwall does seem to be perfect for many types of projects but I have no plans to try it.

Because I can't find the stuff that's why.:)
If it is sold in Western Canada I can't find it.
The closest product I can find is called Greatwall.
It is a fiber based stucco product as well but only comes in basic gray and not designed as a finishing product.

I've just started making half balls with the stuff. It is strong.