I want to screen print a positive silhouette of some physical objects such as leaves and sticks. I plan to lay the items on the screen and spray on the block out or filler.
I've only been able to find brush-on and pen-applicator screen filler. Is there such a thing as spray filler? Or would something else work, like maybe a heavy application of ordinary spray paint?
I don't know of any spray on screen filler but you may want to look into trying some light sensitive photo emulsion. Its kind of like taking a photograph, basically you cover the screen with the emulsion, let it dry, place the leaves on top of the emulsion, and expose it to light. The light will harden the emulsion all over the screen except where the leaves have blocked out the light. You wash/scrub the remaining emulsion out of unexposed spots. Since the emulsion is light sensitive, you'll need to keep it in the dark until your ready to expose it, any room with out windows, or with the light completely blocked out will work. You'll also need to use a red light while working with the emulsion in the dark. I've used Diazo photo emulsion but there are plenty of brands out there. You can find it online or at one of the chain arts and crafts stores. Its not too tough to do, this is how I make my screens.
Here's a link to more detailed instructions from the manufacturer.
http://www.jerrysartarama.com/DOWNLOADS/INSTRUCTIONS/DIAZO%20Instructions.pdf
Typical silk screen "engineering" is performed in such a manner as to provide very high resolution and high fidelity to the original artwork. For the project you describe, dealing with three dimensional objects, you are not going to achieve such fidelity, nor would I expect your objective includes that aspect.
You could use an ordinary spray paint if you do not require to later remove the material from the screen, as is commonly done with conventional silk screening. You should consider to use water based paint to ensure the solvents will not dissolve the screen.
Alternatively, one can purchase an atomizer type or pressure type spray device and use the liquid mask, again making certain that the materials are not going to react with each other.
If you use any type of spray method, flatten your items as much as possible, unless "fuzzy edges" are part of the art work. Use light coats multiple times rather than few heavy ones. You'd want to keep a constant angle from the spray to the art pieces to the screen, in order to reduce the under-spray which would result from spraying under the overhangs of the items.
You mentioned branches (sticks) in the piece. If you care to increase the level of success, consider to sand flat the portion of the branch contacting the screen.
