From what I understand most stencils are put on the substrate side for obvious reasons. Now as I am doing text this would have to be with in mirror fashion.
To date I,ve already screwed up 2 exposures forgetting this.
My question would be could i put the stencil the right way on the squeegee side and still be OK always exposing with the sun.
To get the best detail, the emulsion of the stencil must at the least be even with the bottom of the mesh. This produces the sharpest print image. if the emulsion is thin and only follows the knuckles of the mesh, then you will get 'sawtoothing' or mesh marks on your image.
Typically, we lay the film in position against the bottom of the stencil, 'emulsion to emulsion' -the emulsion of the black image on the film touching the emulsion of the stencil. when compressed this provides intimate contact with no film barrier, and produces the sharpest stencil edge.
if you expose from the top down, and the film has the emulsion on the top, you risk the light undercutting the image through the film, and also through the emulsion of the stencil. the result is the image on the stencil is reduced, or the edges are frayed, or both.
Really, you need to just remember that when you look through the screen from the top side (squeegee side) the type is 'right reading'.
having said all this, you can expose from the top down. I know our friend Michael Motorcycle does it this way - no flies on that guy, or his art. It's just that if you are printing halftones or extreme fine detail, you will not be able to make an exact duplicate stencil of the image on the film.
Just 2 words on the emulsion: more coats more ink you'll push throught the mesh.
Makes tests and diluite A BIT the emulsion, so the stencil will be easier to make (less emulsion=less exposure time=less ink in the mesh=less problems in general!)
I use inkjet films and I've seen 1 single coat on the bottom isn't enough, at least with diluited emulsion, because when I vacuum expose my screens, also if I dry previously them properly, a bit of the black photo ink of my epson remains sticked to the emulsion. If I see the film against the light, I can see thousands of very little pin holes..these pin holes are a pain if you need to re-expose a new screen with the same film, because passes the light.
I believe to know what happens in the vacum, using 1 single coat with diluited emulsion: with the negative pressure, if the emulsion isn't enough to "be the least even with the bottom of the mesh", like said Andy, the ink of the film comes in contact with the un-even surface of the mesh and happens the "patatrack" (mess), remaining sticked, theorically, on the higher part of the threads. This things doesn't happen, so much, with a screen perfectly coated more times, where the next coats make the surface of the mesh perfectly flat for the film.