Reddit mentions of Krylon K05131507 ColorMaster Paint + Primer, Ultra Flat Primer, White, 12 oz.

Use spray paint that also advertises as a primer. It will make no observable difference if it blobs up or runs a bit.

http://www.amazon.com/Krylon-51315-All-Purpose-Interior-Decorator/dp/B001B73WCO?ie=UTF8&*Version*=1&*entries*=0

Don't get too hung up if it's flat white, gloss etc. That only has to do with the amount of diffused reflection versus spectacular reflection. It does not make that much of a difference if the light source is close to the plant. It will make a difference if the light source is further away from a plant where you would want a more specular reflection.

For very high reflectance use white paint with barium sulfate added. It will beat most all other white reflective sources.

http://www.triticeaecap.org/wp-content/uploads/2011/12/Barium_Sulfate.pdf

http://www.amazon.com/Pyro-Chem-Source-Barium-Sulfate/dp/B00JGDLLR8

GW sprays have a very high cost, and people have had numerous complaints about how runny they are.

http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat470015a&prodId=prod1200064

It's always been suspicious that GW is very reluctant to ever call their sprays "primer" even though that's the advertised intent of the sprays. They call them "spray paints" and "undercoat", but never primer... even on the bottle itself.

Even more odd is that their sprays used to be labeled "primer". They dropped that ever since they revamped the cans. The White spray they have on their site is an image of the old can, you cannot get sprays that have that labeling anymore:

http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat470015a&prodId=prod1200062

Again, opinions of course. If you want inexpensive yet good, I've heard good things from Krylon primers:

http://www.amazon.com/Krylon-51315-All-Purpose-Interior-Decorator/dp/B001B73WCO/ref=sr_1_1?ie=UTF8&qid=1367785170&sr=8-1&keywords=krylon+primer

If you want expensive yet (arguably) great, I use and love Army Painter primers. The nozzle never clogs, and the spray is really really strong and clean. I've found that even when I spray too closely and seem like I've obscured the model details, the paint dries and everything is fine.

http://www.amazon.com/Army-Painter-Matte-White-Primer/dp/B004VEO5FM/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1367785212&sr=1-1-catcorr&keywords=army+painter+primer

Here's black.

Make sure you get the "flat" colors (see the bottom right of the picture on the label).

Here's Krylon, it works too if you can find it.

I always use primer. It really helps in all the ways previously mentioned. After some experimenting, I really like the Krylon range https://www.amazon.com/Krylon-51315-All-Purpose-Interior-Decorator/dp/B001B73WCO

Just hold the models about 1.5 - 2 feet away, and go in short bursts. Never let the can stay still while spraying. All you need is a very thin, even coat all over the model. Don't worry if it doesn't look good, the worst thing you can do is go overboard and start filling in details. Shake the can often between sprays as well.

If I do ABS (which I doubt...PETG is fine for what I do) I will vent the fan through the wall into the garage as well as sealing up the cut outs. The fan sucks out the back. For the paint I went with spray cans...2 of this https://smile.amazon.com/gp/product/B001B73WCO/ref=oh_aui_detailpage_o04_s01?ie=UTF8&psc=1 and three of these https://smile.amazon.com/gp/product/B00KZ6LYFO/ref=oh_aui_detailpage_o04_s01?ie=UTF8&psc=1. It is more of a satin finish than semi gloss which I prefer and I lightly sanded between coats. Thanks for the compliment and happy printing!

Honestly, you already know what you could have done that you didn't. That said, they're pretty good for a first timer. If you're going to paint Reaper minis, priming them is key. I have used the Master Series paints that are designed tip work with the Bones line, and even though Reaper says you don't need to prime, you do. I use their black paint on primer, and I have good results. I wish I had a decent camera to take pics of my work.

Enjoy the hobby, and nothing else. Your free time and money just found a new place to go...

**oh! Nearly forgot. If you decide to go the route of spray on primer, I've used with great results, Krylon primer. Buy it local, it's cheap.