How to prepare a plastic bumper for automotive base paint.
Here is how to prepare a raw plastic bumper using only three easy steps. Sometimes we can over complicate preparation on a raw plastic bumper, but it's not as hard as you think.
The 3 Step Process:
On bare plastic you will need a grey pad , then a scuffing paste (there are many good product options here), these have fine abrasives in it. They will clean the plastic and it will help scuff it the same time.
Important: if you over sand this bare plastic it's going to be a nightmare for you, the scratches will swell when you go to put your sealer on and it's going to be bad news. You are going to end up having to sand and prime the whole bumper. Make sure you have appropriate gloves on so you're not putting oils into the finish. You really don't really want to dig too hard, you want to just scuff the surface, so it becomes just a little bit dull. This is going to give you the mechanical and the chemical bond that you will want when the adhesion promoter hits it.
Start off by just wetting up the bumper just a little bit then grab your scuffing paste and put it on the grey scuff pad and you are good to go. Try and go into all the fine nooks and crannies making sure that you have just a little bit of a scratch for our adhesion promoter to stick. This is the most important part of the stage and this is why you'll find that your bumper is peeling if you're not prepping it properly.
Now, there's actually no real reason to prime the bumper and to sand it down at all, we can just use our sealer after the adhesion promoter it and go right to base coat.
It is always a very good idea to go ahead and look over the bumper before you start to make sure you don't have any scratches.
Once you get a good enough scuff you can go ahead and wash it off. After it's been dried it should look it will have scuff marks in it. You'll be able to tell that you've got it nice and scuffed because there won't be any shiny plastic parts.
But let's take a step back here, how do you even know that my bumper cover is a raw plastic bumper? A couple tricks to find that out, find a part of the bumper that is not going to get painted, take some reducer or lacquer thinner and rub in the area right and then go ahead and look at your towel. If your towel is clear and is not marked up with any sort of primer, then you’ll know that this is raw plastic.
Often there are aftermarket bumper, covers or spoiler kits and when you see them, they're really slimy looking and they have a mold release agent. That agent is like butter put all over the plastic so when it's pulled from the mold, it comes out nice and easy. You will need to get that mold release off. There are a couple ways you can do that - you can let it sit in the sun for the good part of the day and let that evaporate off or you can take use a plastic cleaner. If you don't release that or remove it when you go to put your sealer or your primer (or whatever it might be) it's going to fisheye really badly.
Next is preparing the bumper for the adhesion promoter. There are two ways to kill the static.
One way is with a neutralizer. If you're just a hobbyist cost wise this may not be practical.
The second way is to use an anti-static surface cleaner to clean the bumper and kill any static that we have left on the bumper still. Just pump it up and spray it on liberally. We are a big fan of microfiber towels, they really pull off any of the contaminants or dirt and leave the surface nice and dry.
The next step is an adhesion promoter or your plastic primer.
If you use a 1k product you can spray out of a gun, it doesn't need any hardeners and nothing else is mixed into this. Or you can spray it right out of the aerosol which is an easier way to do it. It will dry right away, don't worry if it sweats a little bit. That's what we call it, this will evaporate that and that won't be a problem. Leave it alone, it's good to go, one coat is all you really need.
Allow your adhesion promoter to dry for about a good 10 minutes or so (Always refer to the products TDS). You can go a little bit longer if you want but it does flash off pretty quickly. Next a sealer is going to go on, mix it a little be light. That way you can have good coverage.
This is going to give you a cushion between your paint and it's going to complete the process for preparing your brand new raw plastic bumper cover. Some like to apply this to a 1.4, putting about two coats on back-to-back. So, ready to seal the deal. Go ahead and start sealing, after just a few minutes of spraying the sealer it's going to start to flash and look good. Allow this to flash a good 15 more minutes to make sure it's nice and dry before you hit up your base.
In a really good scenario, you don't have any dirt nibs.
But let's say you do, make sure you're not sanding out the dirt nibs in the sealer. Put on your first coat of base and then you can sand out your dirt nibs with around 800 grit, if it's a metallic. If it's a solid you can go down to maybe 600 grit. But be very careful not to go too rough. From this point treat it like any other job it's ready for base, it’s for clear. (Make sure to use a flex agent with the clear) Lastly, put the clear is on! That’s it!
Please note this is general advice only. If pain persists see your paint professionals.