Confused after float test

[Kenzie]

i have quite a few plastic pieces to paint for my 94 Miata so I ordered some promotor and did a water float test with slivers of each piece. Initially only one of the eight slivers floated. However, after leaving the cup for a few hours I came back to find four of them floating now. Do I trust the initial results or results after sitting?
The really confusing part is that the rear bumper sample floats but the front bumper doesn’t. They’re both yellow/white (TPO?) factory pieces and the material looks the same.
Most of the other parts are cheaper, aftermarket parts that are very flexible and smooth, I’m assuming ABS and polyurethane.

 

[AAE]

Maybe this will help identify what you have. You may want to call Polyvance in the morning for more help.

 

[Chris_Hamilton]

They’re both yellow/white (TPO?) factory pieces and the material looks the same.

That sounds like polyurethane. TPO is usually black or grey. Been a long time since I worked on a Miata so I can't remember anything about them.

Could you post a pic of the covers?

 

[Glen440]

There should be a plastic identifier code on the back side. It's all I use at work.

 

[Chris_Hamilton]

After thinking about it a little I'm fairly certain OEM Miata bumper covers are urethane (PU). Like Glen said above if you can find the codes on the back take a pic and we can tell you for certain.

 

[RosharonRooster]

Is it ever bad to use adhesion promoter? Is it one of those products that are like when in doubt use it anyways?

 

[Chris_Hamilton]

Is it ever bad to use adhesion promoter? Is it one of those products that are like when in doubt use it anyways?

Stuff that doesn't need it your adhesion won't be as good if you use it. Barry told me in the past that his testing showed worse adhesion when used over surfaces that don't need it. PP and TPO are the two main plastics that require it.

 

[RosharonRooster]

Good to know, thanks

 

[crashtech]

Tap water usually has some dissolved gas in it that will come out and stick to anything that's in the glass. Make sure any gas bubbles are knocked of the samples so they aren't being artificially buoyed up.

 

[20mercury]

Tap water usually has some dissolved gas in it that will come out and stick to anything that's in the glass. Make sure any gas bubbles are knocked of the samples so they aren't being artificially buoyed up.

agree with crashtech, and also water will have some surface tension (as to why you can float a needle on water) which might be affecting your test. a couple of drops of dish soap dissolved I think greatly reduces surface tension.

 

[Kenzie]

UPDATE:
I repeated the test using bottled water and nothing began to float after sitting this time so I believe the issue was bubbles in tap water, as suggest.
None of the cheap eBay parts float (they’re not labeled on the back but they’re all really flexible, black, and listed as ABS on their respective listing descriptions ‍)

Main thing that really still has me confused is the conflicting results with the bumpers. The back bumper sample is floating and the front sinks. I’ve cut new samples and it’s the same each time. Both are OEM bumpers (rear is original to the car and the front is from a parts car but still an OEM. I would check the year but the VIN is no longer legible) I’ve also pushed the floating bits down to overcome surface tension and they pop right back up. And, yes, @Chris_Hamilton you’re absolutely correct, they’re PU, not TPO. I got my plastics confused but both are white/yellow and don’t melt. Top bumper is the front (no float) and bottom is the rear (floats)

I’m really stressing getting it right the first time with these since I’m using a custom mix of flakes and probably won’t be able to match the car if I have to respray some pieces that peel.
Everything will be sprayed with white epoxy primer, white base coat, intercoat+flake, and universal clear (all SPI, of course)

 

[crashtech]

It's been quite a while since I worked on a vintage Miata, but I seem to recall the back being different than the front. The one back bumper I remember was white plastic but it needed to be treated like TPO, it didn't sand cleanly and needed promoter. I'd never seen white plastic in a factory bumper before, which made it memorable.

 

[Vanillagorilla]

Is it ever bad to use adhesion promoter? Is it one of those products that are like when in doubt use it anyways?

I had wondered this as well, and just used ad pro on all plastic parts just to be sure (i was unaware at the time that it could cause worse adhesion if not needed. What made me stop using it unless nessesary is that I found that on some parts like cheap ebay aftermarket stuff, when using ad pro, the parts would react to it and cause issues.

 

['68 Coronet R/T]

I did this 1990 Miata and you can see here the different colors of the cracked plastic parts.

I was able to repair everything and if memory serves me right, I did use Adhesion promoter on the plastic panels that needed repair.
It was a long time ago but I sanded with 80 grit for good scratch for the epoxy and then sprayed a light coat of adhesion promoter prior to applying the epoxy.