I should start out by saying it’s sometimes possible to expose a Xerox or inkjet print onto your screen without oil. I think the window you have to get a good exposure is smaller without oil, and you need to expose your screen for a longer period of time. Also if you have anything other than fresh emulsion (sensitized emulsion starts degrading after just a few months) skipping the oil may not be a good idea. Sometimes it works just as well without oil if the stars align, but usually I miss the exposure window and have issues with washout, so only skip oil if you have time and ability to experiment.
There are two ways (that I know of) to make positives from a print-out. One is to print the image onto a transparency. This works great, but can get a bit tricky, especially if you work large. The other, which I am going to show you, is to use a print on paper. I just take my flash drive with my images on it to Kinko’s and have them print oversize black and white prints. You can also use plain old Xeroxes or inkjet prints. If you can, change your print settings to heavy ink, high quality, and the color to black and white or gray scale. I prefer to save the file in the size I want to print in a PDF and have Kinkos print it. This gives better control and detail than a regular enlargement.
Once you have your print-outs, lay your prints on some newspaper. Pour some vegetable oil on the back and work the oil around with a paper towel. The oil will saturate the paper and increase the transparency of the paper. Don’t miss any spots, as this will affect the exposure on your emulsion. If you have another positive to oil, you can lay it right on top of the one you just did and repeat the process.
No worries if you are not going to use the positives right away. I wrap mine in some newsprint and they will stay good for several weeks. You may need to reapply some oil, and keep in mind that the oil will start to smell after a while, and eventually rot your paper.
The positives are now ready to expose onto your screen. You will need to increase the exposure by about 50% over a transparency. The oil won’t hurt your emulsion and will wash off when you rinse your screen after exposure. The oil cleans off the glass of the exposing unit easily with a little Simple Green.