Polishing plastic
Jul 21, 2021 at 4:33 PM Thread Starter Post #1 of 10

Lvivske

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So long story short in the middle of a repaint and following a youtube vid, tried to correct and error by wet sanding (1000,2000 grit) then using polishing compound to get int all shiny again. It works, but its full of surface scratches.

218910331_539010117223619_1820893574473227671_n.jpg



versus the untouched clear coat:

216426497_917922822268092_1192335217215987591_n.jpg


Anyone have an idea why polishing compound would still leave scratches like this? Using an 80% poly cloth from costco, kind I use to wipe down my TV screen and other delicate items, not like I'm using a paper towel...
 
Jul 21, 2021 at 5:33 PM Post #3 of 10

Lvivske

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I don't think its your cloth, but possibly the polishing compound.

https://www.fintechabrasives.com/blog/a-guide-to-polishing-compounds--their-uses/

((Note: I know nothing about this. I just did some light Googling.))

its white (turtle wax polishing compound - light to medium), so under the 'versatile' category

guy on youtube who used 3m fast cut plus, is also a white liquid...


edit: its definitely not the cloth i was using or the polish, i used it all over the other ear with zero downside, it was just THIS one that wouldnt stop looking ugly, im perplexed
 
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Jul 23, 2021 at 8:22 PM Post #4 of 10

cgb3

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its white (turtle wax polishing compound - light to medium), so under the 'versatile' category

guy on youtube who used 3m fast cut plus, is also a white liquid...


edit: its definitely not the cloth i was using or the polish, i used it all over the other ear with zero downside, it was just THIS one that wouldnt stop looking ugly, im perplexed
Think about a "melting" layer, rather than a polishing layer.

(almost) all plastics have a solvent. Find yours, and determine the correct application, and removal to render a smooth (melted) surface.

Alternately, determine the heat at which your plastic melts. Expose your product for X amount of time at just under the melting point, or flash your finish at or above the MP of your product.

Most finished plastic products use many methods to achieve their final finish.
 
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Jul 23, 2021 at 8:52 PM Post #5 of 10

Lvivske

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ok i'm not trying to melt the clear coat into the plastic
 
Jul 24, 2021 at 12:01 PM Post #6 of 10

Chris Kaoss

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For such a job you need finer steps in a cross pattern.
So after 1000/2000 grit, you've to use 3000, last step is a 4k or 6k grit.

The scratches are your sanding marks, not the polishing compound itself.

After that, you can polish to gloss. :wink:

Cleaning the sandpaper as often as you can is the key.
Even the tiniest residue on the sandpaper could cause new scratches on the plastic.

For example:
Plastic parts on a car ( its my business :wink: )

sketch-1627142761550.png
 
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Jul 24, 2021 at 8:41 PM Post #7 of 10

Lvivske

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interesting, thanks

yeah i couldnt repeat the look above when I polished other pieces (so it wasnt the cloth or polish), even doing the same 1000/2000/polish combo on a part with an uneven coat, it turned out fine

but above, i re sanded and polished like 3x and it kept winding up the same, it perplexed me. Possible I should have let it cure an extra day?

I'm currently trying to re-assemble and ive let the clear sit for 24 hours and noticed I was leaving finger prints on it when trying to push the ear pads back on. Was able to polish it out but oh my god I thought 24hrs was enough to harden 1000%
 
Jul 25, 2021 at 2:42 AM Post #8 of 10

Chris Kaoss

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Maybe there's not enough hardener in the clear coat?

Got this behaviour on a BMW M5 when fixing a rock chip with unknown clear.
The clear didn't cure fully.

For such small jobs, i'd use an uv-reactive clear, if you're able to get one in a can.

sketch-1627195204559.png

sketch-1627195236095.png
 
Sep 17, 2021 at 7:56 AM Post #10 of 10

tomb

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I disagree about the sanding grit. Wet sanding with 600 should be plenty fine to get a mirror finish before you use the polishing compound. I suspect you're not using a hard enough grit in the beginning - like a 220 to 400, first. The Turtle Wax stuff you mention is great. I use it all the time to clean up transparent acrylic. If the scratches are deeply imbedded, however, it's not going to do any more good than 600/1000 grit. You need to get the entire surface layer smoothed off, which is where the 400 grit or greater comes in, first.
 

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