Why are you guys reccomending going to grits higher than 600 before painting? This is not a show car, and he's going to have to try and match the existing orange peel on his car with the new bumper, so unless your car has been colorsanded and buffed, the extra work with the abrasive over 600 is a waste of your time, and money.
Here is the basics. There is no free lunch.
First, wash your bumper with a green scotch brite pad and dish soap and water. (Get a clean bucket, fill with warm soapy water, and wash the bumper by dipping the pad into the bucket, and washing the bumper. Use this wash to completely clean and skuff up the bumper's existing finish.)
The bumper when done and dry will be very dull looking. (You have "deglossed" the paint.)
Any cracks in the existing paint?
Any deep scratches?
If you decide to use a filler, make sure you buy one especially designed for use on urethane plastic bumper covers. Some companies reccomend the use of adheasion promoters as well. If you are going to use filler, you have to first provide a surface it will stick to. This will take a grinder and 60 grit abrasive. Do not go hog wild, but just feather the scratch, then clean it with compressed air, spray on the adheasion promoter, and mix your filler. (There is a base and hardner.) Put on a thin coat of filler over the repair area, and let it cure for 24 hours. Then you need to DA it down flat with p200. Then go over it with a flexable glaze, and flatten it down again with p400 or P600.
Prime and block to make sure it's flat. I would use P600 wrapped around a rubber block for this part. Your guide coat over the scratched/repair area should come off nice and even, no high or low areas.
Wash the cover again with soap and water, and a new clean green scotchbrite pad. Dry with compressed air. You should be wearing gloves at this point, and work to keep the cover clean of oil, dirt and even finger/hand oils will cause possible problems if your not careful.
Choose a primer/surfacer/sealer. (Especially the sealer if your going over any old paint.) The surfacer will help to hide any scratches you missed.
Most of the new materials are quite flexable, and do not require flex additive. Any flex additive that crosslinks and bonds with the material is a good idea, but ones that are nothing more than slow solvents are a waste of your good money. You should know that many flex additives will reduce your gloss, but they can make the paint much more flexable, and promote good adheasion to the substrate when applied correctly.
Every paint supplier has a system for their products that will work best if you follow what they reccomend, and I don't reccomend mixing products and techniques from company to company. Pick a brand, and stick with it, and you will generally get the best results.
Ok, you will need two saw horses. Wrap them in newspaper. This is so you can lay your bumper cover up on them, and paint it. Other option is to hang your cover from the ceiling with wires when you paint it.
Wash it again after you have it ready to paint. Wear gloves, and keep oils and dirt off the cover.
Prep with adheasion promoter if the supplies your using reccomend it.
Tack cloth the bumper. (Keep the tack cloth clean, your going to use it again.) Tack cloth is cotton cloth with bee's wax coating the cotton. You open the baggie, fluff up the cloth about as big as your hand, and use it that way. Use a light touch, you just want to pick up any debris, dust, hair or crap on your surface, not push the cloth into the surface, and leave bee's wax on the cover..
Mix and apply your primer.
Tack the cover when each coat of primer is dry. (This removes any dry overspray on the cover.)
Allow for the correct amount of flash time between coats, and between applications of material. (Primer, color and clear coats.)
Mix and apply your color. You should have already done a test panel, and determined the right varient to use for your car. Know that if your painting a metallic, that gun distance, speed of aplication and other variables will change the results of the color you see when it's done. (It has to do with what is called "solvent dome" and it's how the pigment and flake settles out with larger wet drops, or dryer small ones. The larger the dome, the deeper the color, and small dome tends to result in more silver, or lighter colors. This is from the same paint, from the same gun... So be consistant with your gun distance, speed and pressure when your spraying your test panels, and your final project, or the color will be wrong when you get done.) Choosing the right speed reducer here is critical too. A fast reducer on a hot day, and your solvent dome will be so small, the color will be more silver or bright than the color of the varient you thought you picked...
Tack between coats of color. (Again to remove any dry overspray between coats..)
Now, here is the final thing to consider. When you spray over a substrate like plastic, it absorbs solvent, so that tends to make your solvent dome smaller, so you need to put it on wetter to get the same color as paint applied to metal panels on the same day, with the same gun.
So, you have your color on the bumper. Now comes the money shot.
Tack the dry cover. Now throw away your tack cloth. You will not use it again when you start applying clear coat, and you don't need it around anymore. (And it is full of dust, paint overspray particles and other crap you do not want in your clear coat.)
Mix and apply your clear coat. (Most color coats do not have a hardner in them, but some will if you use a flex additive.)
The clear does have hardner, and it will be only good to spray for a specific period of time. Be sure your ready to commit to that time when you get started, or you get to sand this all again, and start over...
Clear has the base, hardner and the reducer. You will need to have them mixed to the right ratio's, or you will screw up the job.
Apply a light coat of clear, and then come right back and apply another for a full wet coat. Allow for flash time, so the solvent and reducers can escape and then apply another wet coat. Repeat this three or four times, and you have all the clear coat you should need. If your going to color sand and buff, you might want to put on up to 6 coats of clear. (You are going to be sanding and removing much of it when you colorsand and buff anyway.)
Most OEM applications can be replicated with 3 coats of clear however.
Getting the paint too thick is worse than having it too thin. You want your color to achieve full hiding, but not much more. Same goes with clear, you want the full 3 mils of clear, but not much more if your not going to sand and buff it. (That removes the orange peel look that is on every car on the road unless that car has been colorsanded and buffed.)
You can get less peel by applying your paint in wet coats, keeping the gun close, and moving at a consistant speed, but not so slow you create sags or runs in the material.
If you apply it with too much gun distance, and speed, it will create more orange peel effect, and the paint might not have as much gloss as you want either. (Think of new Ford trucks, they have nasty peel from the factory, and something like a Caddy for a paint job that is smoother, but still has plenty of orange peel, but has much better shine overall in most cases.)
Good luck, and sorry this was so long.
Personally, unless you really want to learn this business, ask around, and see if a shop will let you work there as a paint prepper, and on the off hours, paint your own bumper.
If you are going to just try this in your garage, and with rattle can products, it's going to look like you did it in your garage with rattle can products...
