Tips for keeping overspray out of cowl?

i took a look at the images at photobucket, and your work looks really nice. i can see why you want to keep the cowl as clean as possible. BUT... the truth is, now that it is sealed up, theres virtually no way to mask it off. You could perhaps feed 2" tape, sticky side up from the first slot to the last pulling it with a piece of wire, i guess, but thats too much work for me. my advice is to keep it as clean as you can until the car is ready for paint, (blow the vent area out every time you think about it) do your paint work, and after the car is painted, but before you hang the fenders, find yourself a 360 degree undercoating wand. you can either undercoat it, or blow some paint into the vent cavity.

You can snake a towel up through the hats and cover the area that will be seen, the towel will have enough weight to keep it in place. Once in, you can spread it out using metal strapping through the vent openings.

I like the towel idea. Here are two other ideas: WD40 or some other light greasy spray inside the cowl will keep the paint from sticking (but shouldn't affect the existing paint in the cowl if it is cured). Then hose it out with a mild detergent to get rid of the WD40 when finished. You should have little or no paint that sticks, and the sanding dust will wash right out. BUT, if you get any on the outside of the cowl, you will have paint adhesion issues. If you are worried about that, you can try to feed aluminum foil into the cowl and flatten it down with a long thin tool. To remove, you may need to catch a tool onto it to lift it up from the cowl floor. It should not be too hard to pull out. I use aluminum foil as an all purpose paint mask and to protect things from overspray. It's cheap and easy to form.
Daniel

DO NOT spray wd 40, or any other silicone lubricant anywhere near an area you intend on painting...these snake oils are a painters worst nightmare!!!