Polishing Plastic Bumpers

Z

ZeroDfx

Member
I recall asking the forum a while back about polishing the plastic bumpers on my BMW E39 5 series, and the general response was to polish like the rest of the car. Someone mentioned using XMT 3 if I recall.
In any case, I went ahead and polished the rear bumper of the car with a 4" pad and Optimum II polish, followed by Menzerna finishing polish, followed by Blackfire Wet Diamond sealant. Seemed to look pretty
good for a while until I noticed the clear coat peeling (see attached photo). My conclusion is that I can't treat it like the rest of the car, and that my pad/polish combo was too aggressive.
So I will now plan on re-painting the bumper. But it still begs the question of how to properly deal with the plastic bumpers. What do you professionals do to allow the bumpers to look as good as the metal?
Thanks
 

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Firstly, sorry about your situation. Not a nice thing to happen, but certainly a learning opportunity for the future....

Secondly, I am not a professional but will say that:

a) plastic does not dissipate heat as much as metal, so I tend to jump from one side of the bumper to the other to try and let the previous section cool down.
b) wherever possible I will reduce the aggressiveness of the combo I am using on the rest of the car when dealing with the bumpers or other painted plastic sections.
c) sometimes changing machine helps with heat control too - for example I have found the Flex 3401 gets a lot of heat build up on the FACE of the pad (and thus the paint) whereas a traditional free floating spindle DA (I use a DAS 6) tends to get heat buildup at the BACKING of the pad (thus the paint runs cooler)
d) ideally you would want a PTG capable of measuring the paint thickness on plastic before deciding on what combo to use - in the absence of such a gauge, asking a few questions of the owner around any previous polishing that may have been done could help. Also, spotting things like buffer trails/holograms on the bumper will be a good indicator that the previous guy polishing went AGGRESSIVE so you would then be even more gentle.

If I may ask, what machine were you using, what pads and how many passes did you make over the section.

It may also be that the bumper had been aggressively polished previously and you just happened to be the last guy in the chain who was left with little to no paint to start with.
 
Thank you very much for your thorough response; much appreciated.
I was using a PC7424 with a 4" orange pad with the Optimum II to start. It's a fairly small surface area to begin with and difficult to get close to the trunk side and the trim side, both of which I had taped off.
I did left-to-right passes and back again until the polish disappeared and lightly hazed. Wiped clean and moved on. Repeated with 4" white pad and Menzerna FP II; applied and removed BFWD by hand.

Here's a link to the original conversation about an earlier car back in 2008: http://www.autopia.org/forums/car-detailing/136576-detailing-plastic-rubber-bumpers.html
 
So this paint started peeling 7 years ago after your initial polish or are we just seeing what's degraded over the past 7 years?
 
I detail as my job. While I am sure others have more years of experience I'll share what I find the most safe and effective way. I typically use an Orbital Buffer on the rest of the car or the Flex 3401. The Flex 3401 seems less gnarly against plastics as the orbital but it still heats things up quite a bit. For that reason I usually try not to use them for plastic bumpers and mirrors unless I absolutely have to. Then I do it on the slowest rotating speed and move as fast as possible while still removing scratches. I constantly check the temperature of the surface. If I can get away with it I just switch to a regular old DA. I use the same compound and a Meg's foam cutting pad. I just take my sweet time and use a synthetic sealant as the LSP for all painted surfaces.
 
Just don't be as aggressive, which includes not abrading it as often. And yeah, the way plastic handles heat means you have to be even more gentle than with metal panels.

And FWIW, the only actual "gotta repaint that" damage I've *EVER* done was with a PC/4" polishing pad/mild product (3M 05937). Not with the rotaries or the Flex 3401, not with my harsh compounds, but with that combo.
 
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